While under normal circumstances, you'd be right, as many Asian movies get subtitled and released to the rest of the world, like Horror movies and Kung Fu flicks, you also have to remember that most English speakers have this weird mentality that cartoons are for kids. Which also goes hand in hand with the idea that kids are idiots and can't read.
Of course, you could say that some anime is clearly not meant for kids, and you'd be correct, and everyone around you would nod in agreement, but even so, people still can't get rid of that notion in the back of their heads that cartoons are meant for kids, and kids can't read, regardless of the actual content in the series.
Think of it like this. Underage b& often complain about the lack of voice acting in Zelda. People automatically assume that they're just dumb shits who can't read.
So, even though it would probably be easier and better for companies to just sub series and not bother with voice actors, it's something that you just can't drop so suddenly without changing people's way of thinking.
Or that's my two cents on it anyways.