Because AAA games cost more to make than blockbuster movies.

They take more art than a blockbuster movie, they take more script than a blockbuster movie, they take more motion capture than a blockbuster movie (except maybe the Avatar series which are basically all-CGI/motion capture), just everything is so much more than what you need in a film plus the need for compelling gameplay with a satisfying ludonarrative.

The massive amounts of capital needed to make one to begin with literally preclude studios that aim for this style of game from doing anything but submitting to large publishers who will finance them. Because the resources required to make that type of game are just that much.

It’s part of why Larian did Early Access for BG3 so they could make money during the development to help lengthen their runway and lower their burn rate. They still had to cut a huge amount and just release as-is. The Upper City got cut entirely as did the return to Avernus and with it Karlach’s actual happy ending, all of her current canon endings are technically fail-states for her storyline.