There's a blog series on film scoring that I can't seem to find again right now, but in it the blogger (who composes and conducts orchestras for film scores) mentions that key signatures are never used for film scores - all accidentals are explicitly written in. The reason is that when a film score is recorded, the musicians are seeing it for the first time that day. It is 100% sight read by the whole orchestra.
The blogger's workflow is to get to the recording studio about an hour ahead of time, and that's when he first sees the score. He reads through and looks for areas that may cause trouble or where perhaps the composer wrote something that won't work exactly as written. When the musicians come in, they see the score for the first time and perhaps minutes later will be running it down with him conducting. There is no prep time but the musicians are professionals who sight read very well, it's just helpful for them to not have to remember the key signature. According to the blogger, most passages will be recorded and done in three or fewer takes.
When you combine the intense, immediate sight reading with the tendency of film scores to have frequent modulation, modal mixture, and chromaticism of all kinds, it starts to make a lot of sense to have all accidentals next to the notes and not deal with key signatures.
As with most modern music, we do not use key signatures in film scores. Even if the score is completely tonal. It is much easier for the players to have each accidental labeled. If we did use key signatures, players would end up writing in a lot of courtesy accidentals, as they will be sight-reading and they do not want to miss any.
Who is this guy? From his bio page:
Tim Davies is one of the busiest conductors and orchestrators in Hollywood. His film and TV credits include La La Land, Trolls, Minions, Ant-Man and the Wasp, Empire, The Peanuts Movie, The Muppets, Lego Ninjago and Frozen. He is also the most prolific orchestrator and conductor in the world of video games having worked on multiple titles from the God of War, Infamous, Sims, Resistance and Batman: Arkham series and Marvel’s Spider-man.