I suppose it's perfectly possible that the U.S. has similarly high-quality programmes, but if so, they don't advertise them nearly as well as the U.K. To a non-American non-British person like myself, U.S. programming means sitcoms and exciting "popcorn flick" movies (not that there is anything wrong with that---I enjoy many American programmes), while U.K. programming is more cerebral or interesting (Inspector Morse, Doctor Who, Yes Minister, Red Dwarf, etc.)
You're kidding, right? Just for starters, I have a hard time believing that you've never heard of HBO. I think it can be safely argued that the quality of American television series is without par right now, and has been for the last ten years. And even if you don't agree (but you should), the quality of independent movies in the United States is fantastic as well. There are many reasons for a British actor to want to work across the Pond that don't include money.