Because many Americans- specifically religious conservatives WANT such censorship- they are the reason it is done- because if TV stations show bare butts on the channels their kids watch they complain to the FCC about it.
When Janet Jackets' nipple was briefly visible on the 2004 Superbowl half-time show millions of people complained to the FCC and their congressman about it and tough new rules were passed requiring time-delays and "kill-buttons" on live broadcasts to prevent unexpected profanity and nudity.
So it's just easier to not show anything that offends anyone- that way you don't get complaints.