Also, aside the already mentioned above, it could also can come down to budget or timing. If an ending is far too ambitious, it may exceed the remaining funds available and they have to either find additional funding or rewrite. If they are under pressure to complete before a date, a rewrite may be necessary and the already filmed ending might be scraped entirely.
Monty Python's Holy Grail did this, but instead of reshooting, they just took a left-turn and changed the ending to avoid shooting a costly and massive battle scene.
The movie was supposed to end in a large battle scene between King
Arthur’s knights and the outraaageous French forces, but it quickly
became apparent the production was out of money and couldn’t afford to
shoot the scene. The solution? Just as the siege is about to begin,
modern-day police enter the scene and shut down production,
investigating the murder of famous historian ‘Frank’ earlier in the
As such, there might be abandoned endings that never get full treatment, might have no CGI added or only simple CGI for placement.
Another reason might be to cut down the length of the film:
...the transmission tower for the plans was separate from the main base
on Scarif. To transmit the plans, they had to escape and run along the
beach and go up the tower. In cutting the film, it just felt too long.
We had to find ways to compress the third act, which was quite long as
it was. And one real, fast, brutal solution was to put the tower in
the base, so they don’t have to run across the beach and do all of
that stuff to get there.
I've seen it happen firsthand working on TV shows and movies, where they completely scrap the scenes, reshoot, or rewrite. I've got alternate versions that the world will never see floating in my brain, and if I watch the scene replacement, I can't help but to think of what it was.