There are lots of different reasons, but a central problem is the point of view is simply different between movies and video games.

Video games as an emergent narrative art form are unique in that they are the only narrative medium to be truly 2nd person. The player characters is you. You are Leon Kennedy, Laura Croft, Mario, Link, ect. Think about Telltale’s games or Life is Strange. The primary gaming mechanic is simply making choices.

In video game development there are endless discussions about how to grant player agency. Because good video games lean into the fact that the player character is you.

As a side effect, this means that video games can have significantly thinner characterization. What do we know about Mario? He’s a plumber, he Italian (and there’s canonical disagreement if he is Italian or Italian-American), and he’s heterosexual and in love with the princess. A lot of this is technological. The technology to create 40+ hour characterization heavy games simply didn’t exist for much of video game development. The history of video games has been one of accomplishing a lot with very little. Take a look at Street Fighter. All of those characters are memorable and well rendered using little more than a physical look, a geographic location, two or three dialogue lines, and a tiny blurb in a player manual. But we all know Chun-Li.

The reason that we accept thinner characterization in video games is that we fill up the player character with ourselves. In some cases, that is very literal. In a Bioware game the player determines the character. Same thing in Fallout. But even a game like The Legend of Zelda where the game is always marching towards a pre-determined conclusion the player is still controlling the character and forging an emphatic bond with it.

Movies and TV are third person, and mostly third person limited. A person can’t simply fill in any gaps with themselves when it comes to movies. There isn’t that instant emphatic bond. And, as a consequence, movies have a more visceral distancing effect.

Take this franchise:

Literally nothing about it makes sense. The Animus machine doesn’t make sense. The Templars don’t make sense. The Assassins don’t make sense. The whole plotline about aliens doesn’t make sense. I am not saying they aren’t fascinating mythology, but if you actually sit down to puzzle out how any of this works it will be an exercise in futility. What’s important is that you are instantly immersed in this world as an active agent in it, you aren’t thinking how ridiculous a concept the Animus is, or why the Templars are spending days and days watching an assassin travel around doing things completely unrelated to anything they could be interested in.

However all those barriers suddenly go up when it is a film adaptation. You aren’t instantly a part of the world in the most visceral sense. So the world and the characters that inhabit it have to be able to stand entirely independent of that player agency as equally fascinating. And, in Assassin’s Creed’s case they just weren’t.

In order to make a good video game movie, a studio has to take the player out of the game and still find enough stuff in it to be worthwhile. It’s entirely possible, see Castlevania, but it requires extraordinary work.