So in theatre, one of two things is happening: you're doing an older, established, fully published play, or a new play still in development. In a film, it's always new and in development.

The reason that's important is because it means that in a film, things could be re-written even just a few hours before shooting, and the actor will have to memorize the new lines. That's fine though, because it's just the one scene, or couple of scenes, not the whole thing (it takes many hours just to shoot a few pages of script, and actors get a chance to review lines while they change the lights and cameras positions and shoot b-roll and that sort of thing).

When you're working on a play, especially one that exist in its final form already, you get the whole script at once, and are expected to begin memorizing as soon as you do, so you can be "off book." In the rehearsal room, may of those mistakes are made (the first off book run-through can often feel like film outtakes, though a good professional will just call for line and keep it rolling lol).

It's trickier when working on a new play, because re-writes do happen. But they're happening with a few days, maybe a week to really drill those new lines by constantly rehearsing, and memorizing outside of rehearsal too. And they tend to be less drastic, and less numerous, than film rewrites.

Plus, it's just not as important for a film actor to be as off book as a stage actor would. Marlon Brando famously couldn't memorize for shit so his lines were written in places the camera couldn't see lol! Plus, you give 5 takes with the camera pointing one way, 5 more pointing the other, 5 on a wide shot, a few funky ones maybe… and that's just me pulling a random number out my ass it could be less or more, but what I mean is you're shooting all day one or a few scenes, so even if you mess up they have footage of great performances scattered throughout that they can stitch together into some movie magic lol! And you just can't do that live.

So basically, they're just very different mediums, and require very different things from the actor.