I believe that, historically, it’s probably because of where the two industries came from. Hollywood workers (actors, directors, writers, camera operators, set builders, costumers, etc.) have a long history of having union contracts with studios to work and get paid on a per-project basis likely since that’s how business was done via the film industry’s ancestor, the theater. They’ve been doing per-project work for over a hundred years and that’s become the norm. Game development evolved from software development, which has existed without a strong union presence or a per-project basis. This pushed the software companies to hire their workers full-time and provide them benefits and perks as a means of attracting and retaining them. Industry overall pay structures are quite different from each other.
This is, of course, my own conjecture. I am no labor historian, so I wouldn’t be able to say definitively or point to any primary sources.
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