In short most of them are Wessler's friend and second its due to Wessler's attitude of “Wait for them. Shoot when they want to shoot. Guilt them to death.”
For long read the below mentioned Wikipedia summary
It took years for Wessler to recruit his actors for the film. He says
that he was turned down plenty of times because all the actors were
asked to work for scale.
He says. “The truth is, I had a lot of
friends who were in this movie. And if they didn’t say yes, this movie
wouldn’t have gotten made." In end, most of the actors were willing to
take part because the film only required a few days of their time and
often allowed them to play a character outside of their wheelhouse.
Hugh Jackman was the first actor Wessler cast. He met the star at a
wedding and then called him some time later and pitched him the short.
The actor read the script and agreed to be a part of the film. “He
called me back I think 24 hours later and said, ‘Yeah I wanna do
this,’ which I think is, quite frankly, incredibly ballsy. Because you
could be made a fool of, or you could look silly, and there will be
people who say, 'That’s crazy; he should never have done it."
After talking to the multiple agents of Kate Winslet, she eventually
agreed to take part. The Winslet-Jackman sketch, that was shot shortly
after, became the reel to attract other A-list stars.
John Hodgman, who plays opposite Justin Long in one sketch, signed on
with no knowledge of the project. Hodgman said, “I got an e-mail from
Justin that said, ‘I’m going to be dressing up as Robin again. Do you
want to dress up as the Penguin?’ And I said yes. Without even
realizing cameras would be involved, or that it would be a movie.
Justin is one of the funniest people on earth.”
Richard Gere, said yes — though he wouldn’t be available for more than a year. So Wessler
waited him out. He thought the idea of his sketch was too good. Gere
eventually called Wessler and told him he was free to shoot, on just a
couple of conditions: They had to do it in four days, and they needed
to relocate the shoot from Los Angeles to New York.
It didn’t work on everyone. Colin Farrell initially agreed to be in
the Butler leprechaun sketch — as Butler’s brother, also a leprechaun
— but then he backed out and Gerard Butler did the sketch by himself.
Farrelly says that when he approached George Clooney about playing
himself in a sketch (the gag was that Clooney is bad at picking up
women), Clooney told him “No fucking way.” There was to be a sketch
directed by Bob Odenkirk that starred Anton Yelchin as a necrophiliac
who worked at a morgue and had sex with the dead female bodies that
was shown at a test screening of the film, but was cut out of the
final film. Producer John Penotti said that the sketch will be seen on
the DVD and Blu-ray of the film.(Source: Wikipedia)