First of all, David Benioff and Daniel Weiss are not only the writers of the show - they are its creators and showrunners, meaning they have a lot more influence than mere writers.
Apart from this, the source material comes from the independent author George R. R. Martin (also co-executive producer of the show), who put his full trust in them, and seemed accepting of the outcome of their free reign:
George’s willingness to pull back his level of involvement is
commendable for an author. The level of trust he puts in David and Dan
is incredible, given his lack of a final say over the product. Many
authors will want total and complete control over their work, but
George, having worked in TV in the past, understands the limitations
of adaptation.
source
As he has not yet written an end to his own storyline*, this has affected the plot and duration of the TV series:
The first four seasons of Game of Thrones were written in conjunction with Martin, after which he "stepped down from writing episodes of the series" (more on that here), because of two reasons:
Martin's storytelling would take longer than planned, as he decided that he didn't want the five years gap in his books he initially had planned on:
After the first major plot arc resolved in the third book, A Storm of
Swords (seasons 3 and 4), Martin planned to skip the story ahead five
years. But he couldn't make the gap in action feel true to the
characters or the world, so he eventually decided to write his way
through those five years instead.
Soon, [Martin's] garden was overgrown, the projected length of the series kept expanding, and the books stopped coming.
source
All the while, the televised series started to catch up on the source material:
So it wasn’t until 2014, ahead of that fourth season (covering the
back half of book three), that concerns about Martin’s books being
left in the dust began to really take root. “I’m hopeful that I can
not let them catch up with me,” Martin said in an interview with
Vanity Fair at the time, hoping the show would spend a fifth, sixth,
and seventh season adapting books four and five, by which time he
would have finished book six, for another season or two of breathing
room. The idea was that he might get A Dream of Spring done before the
show got its say.
Martin’s mindset here is
revealing: in his mind, the show was going to run far longer than it
actually did, telling a story at the same level of detail as the
previous seasons, and as his novels. After all, that’s how the first
seasons worked, and he’d always had the time to progress at his own
rate.
Obviously, that wasn’t the case, and following season 4, Game of
Thrones started to blitz through Martin’s remaining source material.
Season 5 ate up most of the plot of A Feast for Crows and A Dance With
Dragons, largely by sticking to the action and avoiding some of
Martin’s more meandering plots. And while Martin tried to get The
Winds of Winter out before the sixth season of the show surpassed the
novels, he simply couldn’t hit the deadline.
source
The final season had its own specific problems: Benioff, Weiss and Martin tried hard to keep the ending of the series distinct from that in the books, and had sketched the final season years beforehand:
The final season had been discussed with Martin in advance, to prevent similarities between the TV series and the as of yet unwritten books, so as to not spoil the books for future readers:
How much of this season is from discussing the ending with George R.R.
Martin?
BENIOFF: [The concern] used to be that the books would spoil
the show for people — and luckily it did not, for the most part. Now
that the show is ahead of the books, it seems the show could ruin the
books for people. So one thing we’ve talked to George about is that
we’re not going to tell people what the differences are, so when those
books come out people can experience them fresh.
source
Those discussions concerning the final season had already taken place years beforehand, determining an overall structure:
How long have you known the broad strokes of the final season
storyline?
BENIOFF: I remember the two of us talking in season 3.
WEISS: We’ve known the major beats for at least five years. [..] The motivations behind each scene are something you’ve been thinking about for five years.
source
All of that lead to Benioff and Weiss having to fill in the gaps of the rest of the series - manoeuvring between the source material and conversations with G. R. R. Martin, plausible story arcs between the two (semi-)fixed points at the beginning and the ending, and the demands a TV show and its fans had.
This all
left Benioff and Weiss in their own, uncharted waters. The show
had to go on, and while they could work with Martin as much as they
could, they were going to be the ones to pen the ending, especially
after Martin stepped down from writing episodes of the series after
season 4.
Part of the problem was simply in what George R.R. Martin has given
the showrunners. Per Martin’s own admission, Benioff and Weiss “know
certain things. I’ve told them certain things. So they have some
knowledge, but the devil is in the details. I can give them the broad
strokes of what I intend to write, but the details aren’t there yet.”
Simply put: Martin couldn’t help Game of Thrones stick the landing,
because he himself wasn’t positive how he’d put the pieces together.
source
For the next couple seasons, showrunners David Benioff and D. B. Weiss
tried to take over management of Martin's sprawling garden,
simplifying and combining character arcs with mixed results. Then,
with the start of season 7, they shifted their focus from telling the
unfolding story of an entire world to concluding a particular tale set
within it. They gave themselves a fixed endpoint — 13 episodes to the
finale, and no more.
source
Looked at through that lens, the inconsistency of the last few seasons
— and season 8 in particular — makes a lot of sense. It’s practically
a miracle that Benioff, Weiss, and the rest of the writers were able
to give viewers anything resembling an ending at all, given their
self-imposed time frame. Martin has been telling fans for years that
good, rich drama takes time. And the show didn’t have enough of that
time, given how it compressed the series’ conclusion.
Game of Thrones still likely wouldn’t have run for a dozen seasons.
Martin’s story is too complex and internal to fully fit on a screen.
His dream of taking three seasons for books four and five was
unrealistic. Compression was always coming for the story on Game of
Thrones. The only question was whose story would be crammed into the
time the show had left — Martin’s, or someone else’s.
source
Unlike Martin, Benioff & Weiss
weren't trying to resolve every character arc or pay off every last
bit of world-building. They knew the destination Martin had in mind,
they understood the dots they had to connect to get there, and they
wanted to maximize fan entertainment along the way. Then, presumably,
they asked themselves questions. What big set pieces did they want to
deliver? What surprises could rival the greatest twists of the show?
Which of the remaining conflicts would yield the best drama, and which
onscreen pairings would bring the most emotion? What did they think
we, the audience, wanted to finally see before it was all over? It was
a Game of Thrones bucket list. And once they had that list, they
needed to maneuver the characters into place.
source
So, ultimately, apart from being the showrunners and creators, David Benioff and Daniel Weiss had the most possible insight into the source material, had an idea of the ending based on that knowledge years ahead, and had discussed at length the ramifications and consequences of their show with George Martin.
As a side note, I can't find anything on contractual or other obligations towards Martin.
* Even though "Actor Ian McElhinney, who played Barristan Selmy on Game of Thrones, claimed (falsely, it appears) at a convention called Epic Con back in April that Martin has already completed the final two books, and he agreed to wait and publish them after the show finished". Martin reacted negatory.
