Two major reasons:
1) The number of licenses the FCC grants are manageble from an enforcement perspective. There are (I'm guessing) probably less than 10,000 broadcast licenes. Let's be generous and say I'm really, really, really bad at math and say I'm off by a factor of 50, and there are really 500,000 licensed broadcasters, or 10,000 in every state (there aren't). That pales beside the massive, massive number of web pages which easily goes into the billions.
2)As if that weren't enough, the interenet is global, not American. I'm a click away froma German, Swedish, or Kenyan server than isn't bound by U.S. law.
All web pages have to come from servers. There could be other ways devised to enforce it in the US.
Your point about foreign sites I did think about and don't have any good answers for regulating that, but there could be some ways to block them rather than subjecting the US to a set of international laws, some of which may not be in our best interest.
Despite filters and spam traps by my isp, the regular spammers, porn spammers, and scammers constitute the majority of mail in my inbox every single day. I guess it goes with the territory.