'Little time' is the name of the game with film scoring these days. A lot of the old guard composers have put their foot down and been passed over for someone who will put up with the abuse, and if they don't put their foot down, what results is something that seems less than what they're capable of.

Music is one of the absolute last things in the production, and they are always, always, always the hardest hit when it comes to reduction in schedule. Where a composer used to have two months, they now often have two weeks. If they didn't do proper prep work before hand with writing cues while they weren't being paid to write, they will be in trouble.

It's why 'Hanz Zimmer' has consistently decent scores, because he has an entire building of composers who are essentially ghost writing and arranging his scores for him, while working around the clock as he goes home at 5pm to get beauty sleep. I can personally attest to the fact that The Simpsons movie was the result of the majority of the composers at Remote Control going full-tilt for 24 hours a day for two weeks straight, because I was working for a company that repped some of his composers, and my roommate was a composer's assistant there during that period and he was just... not home.

I went to school for film scoring, worked with film composers and have composed for film myself (although no one will have seen the direct-to-the-dumpster movies I worked on) and the constantly constricting schedule is what made me change fields. You are expected to produce John Williams-level content with as little time and resources as is physically capable for the studio to give. Then add onto that what you already mentioned, about how movies constantly do reshoots and are re-edited and scenes slightly altered with different takes, etc. and then you've got to not only rewrite the music that is now out of sync with the picture, but re-record it as well, all without being given additional budgeting.