It's all very simple, Americans go watch sports that are exciting enough to be entertaining on their own, we don't come to the stadium and have to provide our own entertainment

EDIT: Also, in American Football at least, the fans can actually impact the game by participation, and it's not some "intimidation" garbage that never works. If fans are loud enough when the opposing team is on defense, it's difficult for them to communicate in the huddle, and makes it impossible for them to communicate on the line of scrimmage. If you have a good crowd they can cause several false starts in a game.
Meanwhile, you want your crowd to be reasonably quiet while your own team is in the huddle or at the line.