because if anyone can make a movie for $50k and show it to the masses and make 200mil then the studios would lose their controlled market. and since they have no way of knowing which indie film will be a success they can't justify marketing all of them and losing money on 50% of them.
if they buy all 100 movies and distribute all of them, only 20% will be successful, if they buy all 100 movies and only distribute 20 of them, they have a chance for 80% of them to be successful since there is that much less actual content for people to see.
makes sense from an accounting point of view right? well business is the only thing they really care about.