Because if fans want to see people who just enjoy playing basketball and are reasonably competent, they could watch college ball. Fans watch pros to see people who are insanely skilled and owners know that people aren't going to pay big dollars for tickets and merchandise for a cut rate product.
I think both groups are "shooting themselves in the ass". The players are giving up millions and the shelf live of a basketball player isn't long enough to blow off a year of earnings. (On the other hand, after the first few million what are you going to do with it?) The owners are looking at a investments of close to half a billion dollars
that instead of generating $100+ million in revenue is instead bleeding cash just to maintain overhead. The fans are looking at millionaires fighting with billionaires and wondering way they bother with the sport. It seems like a compromise should be available given that just keeping the status quo is a win for both sides.