White people cast into ethnic roles?

Why are there so many films out there where white or half white people are cast into ethnic roles? Do white actors really sell THAT much more? For instance, the Gandhi movie, the dude who played Gandhi was half white. He actually looked the part though, so that was okay, but then in Partition, only one of the three main characters had any indian blood in them at all...and that guy was half too. And don't even get me started on 21...in real life the team was all asian, and the movie version had two asians playing only slightly important roles. I'm not trying to start a flame war or anything, I know white actors sell better, but you'd think that the main character of a movie called "the last samurai" would at least be japanese! Why can't they just make them a little more realistic?