In my defense I'm also critical of Japan's xenophobia (Pretty much anybody who has lived in Japan can attest to this attitude still being present). I guess I just tend to hold at least the United States to a higher standard than that for obvious reasons, not to mention Hollywood is basically in the middle of the US Asianland. Asian people are still a pretty small minority in the US but I somehow doubt you'd be as hard-pressed to find one as opposed to finding a white or black person in Japan.
It's true that I would definitely prefer a decent actor above just casting an Asian for Asian's sake, but it's not like that's what usually happens. I somehow doubt Justin Chatwin was cast as Goku or Kristen Kreuk was cast as Chun-Li because they were just so perfect for the roles (I was actually a bit more accepting of the kid they got to play Aang because he was the appropriate age and knew martial arts, though they kinda screwed that over by not getting appropriate people to play Katara/Sokka/Zuko). If you wanted to find a good Asian actor there's plenty to choose from. I don't think it has anything to do with not being able to find an actor- but I also don't think it has anything to do with "racism" most of the time, as in the casting director just doesn't like Asians or whatever, but rather that studios arn't willing to take risks and they prefer to have their lead be a straight white American male (Basically making it come down to $$$ as with most things). I think someone once said if a show ended up being a hit and happened to have an Asian in the lead role it'll probably happen a lot more often, which is why it annoys me when they don't cast Asian lead actors when it would've otherwise been appropriate.
Bringing it back to Monster specifically, I feel that Tenma's cultural background plays pretty heavily in how he's characterized as well (not to mention the several times in the story the only reason it made sense for people to find him was because he is "that asian doctor").