hollywood is a business because for some reason movies cost money to make and they try and give the public what they want to obtain that money. This is the way hollywood has worked for nearly 100 years. So you pretty much get a lot of shit and some stuff that will endure. The actors you named undoubtedly turned out some hot garbage in their time and probably knew it was going in. And remember when you praise the state of film in other countries that only the cream of the crop makes it over here. Its likely they have the same hit to miss ratio as we do. Unexperienced actors also tend not to have any range beyond roles that resemble their own lives.
Plus even an actor you named as bad, Matt Damon, has made some pretty ambitious films. Take a look at "Gerry". As for Soderbergh the money he made from oceans 11 was used to produce a film called "Far From Heaven".