Why do films play fast-and-loose with accents?

One of my biggest personal pet peeves is taking Welsh accents away from people who have them. Like, in the English dub of Howl's Moving Castle, when Christian Bale does Howl's voice and sounds colorless and American. This drives me nuts because Howl is Welsh. Kind of like Christian Bale. Or Ioan Gruffudd. Who sometimes plays English people, which is okay. But then in Fantastic Four, he also must do a stupid American accent. I guess Mr. Fantastic is technically American (that's comic book experience I don't have), but I don't think it would be narratively disastrous if he were Welsh.

So there's that. And then there's the other direction. And here comes the long-awaited Peter-Jackson-directed rage.

Sean Astin? Couldn't we have done better than that? Isn't there some questionably talented British actor who looks like a potato? Because then at least his accent wouldn't wander over all of the British Isles and much of the eastern United States. It really kills the moment when, in the midst of all the grime and Orcs, there's a random Yorkshireman out of nowhere. Man, it's the worst. I pass over Elijah Wood.

But I cannot pass over Viggo Mortensen. The man sounds like a dying cat is lodged somewhere in his vocal cords. And laid over that is this quasi-British proto-accent that drops the Rs but does damn-all for the vowels, because no one has yet had the decency to kill the damn cat.

Now, I understand that sometimes the main thing for a rôle is the appearance. But we complain when people are just terrible actors. And, actually, talking in a remotely plausible manner would be part of being a good actor. Please, people.