I have a pet thesis that I've never bothered to develop empirical data to either prove or disprove. The thesis is that most people in entertainment today are not there because they are especially good or were discovered, but because they are/were related to someone in entertainment who gave them an "in."
If so, this leads to the reason for the treason. They feel guilt because they have never had to do anything to get to where they are. They get over their guilt by taking shallow but extreme positions, positions that allow them to view themselves as champions while having no cost to themselves.
How many of the above named ultra-left wing, America hating "entertainers" owe their celebrity to a relative?
Bingo. I work in a showbiz-related field (radio), and my wife is a stage actress and singer, so I've met a lot of celebrities in my time, including many big name actors and musicians. You've pretty much summed up what I've been saying about them for 20 years. But every so often, you get a surprise. You should hear Johnny Rotten go off on one of his hilarious Libertarian rants. He sometimes sounds like a Cockney Dennis Miller.