The difference is that when a movie is completed it can keep making money for years and even decades. Back to the Future is still a viable moneymaker for Universal. Even films that flop at the box office can earn money through various means. When movies age they don't lose their relevance. People will watch A Christmas Story on TBS or buy a Blu-ray of Terminator 2. Video games, though, age like computer software. After a few months they've essentially exhausted their entire revenue stream. And they generally can't be resold or repackaged as-is unless it's bona fide classic like Super Mario Bros. 3, and even then it won't sell for more than $5 or so. Budgets need to reflect this reality.
It's specifically because games are competing with Hollywood that this mess started. Back in the mid-90s a budget could only be so big, regardless of what a game's vision was. A flop could only do so much damage under these circumstances. But the shackles came off and the beast ran loose. With those technical limitations no longer an issue and 15 years worth of unchecked growth, here we are in a state of unstable equilibrium in which the majority of publishers are operating under the constant threat of total collapse. It's unsustainable.