What's With The Brits Playing Americans?

Actually, I think Liam Neeson is Irish maybe? But is it me, or should a role like MacArthur go to anyone out there willing to portray him? A figure like that should be done by a good American actor, shouldn't he? Harrison Ford, James Cromwell, hell Leonardo DiCaprio. I don't get why all these Euro's are playing historic American figures now? Are they that much cheaper than American actors of as great or lesser talents? I'd love to see a movie about Viet Nam where all the American's are played by British actors, and all the Vietnamese are played by American actors. LOL