As mentioned, whatever the public will buy is what gets made the most. We are almost always the cause of all problems. Of course there are LOTS of really good movies being made all the time, they just don't have big advertising budgets so most folks never hear about them and they probably usually barely break even, if that. Once in a while one will break out.
The biggest single contributing factor, it seems to me, is that it's a completely proven phenomenon that people will give a movie a chance if it has someone in it they recognize or like, whereas one with no recognizable actors will just get browsed by (I was going to say walked by but no one walks by movies anymore.) That is the starting point of a downward cycle.
So now, the bean counters are thinking, well, that means we have to have really well known people in every movie. And of course really well known people don't come cheap. They may be almost as big an expense as the actual production costs of the movie (maybe bigger in some cases?) And those folks don't want to work on a dinky movie with a non-legendary director. Now, the movie that could have been made for $5M cost $100M to make. So now it's a huge gamble and if it tanks it could be a disaster. So, we can't let it tank. So we go over it with a fine toothed comb to make sure that it's as completely generic as possible, that every known standard plot element and cliche is incorporated, that nothing risky is done, that the story arc is perfect and completely and happily wrapped up at the end. We pass it through five sets of script writers, each one of which makes it more generic. Then we spend another $50M promoting it, with materials that are covered with pictures of those well known people making one of their three standard faces, extra points if holding a gun and scowling or half naked (or both.)
Then, of course, people see the deluge of content with pictures of well known naked people scowling and holding guns and they go to that movie instead of the incredibly creative, quirky $1M movie with people they've never heard of. And the cycle continues.