Why do some actors have a terrible accent in movies?

It's a big question because there are tons of different reasons. First of all, sometimes it's just that the actor isn't from the country whose role they're supposed to play, and doing a convincing accent is way harder than people think. Even someone really talented can struggle if the accent is really far from their native language. Then there's also the production side: directors and dialect coaches don't always have unlimited time to work on the accent during filming, especially when you've got complicated scenes to deal with at the same time.

And honestly, there's something people forget: sometimes it's a deliberate choice, or at least an accepted one. An actor can nail the accent but make it a bit exaggerated so the character is more readable for the audience, or on the flip side they might decide it's not the most important thing for their performance. Sometimes producers also prefer a nuanced performance with an okay accent rather than a flat performance but with a perfect accent. It's compromises that happen behind the scenes.

You also gotta remember that a lot of people watch dubbed versions or with subtitles, so the accent problem is really only an issue for people watching in the original language. And honestly, if the actor is playing their character well, the accent that's there often goes unnoticed.