Because civilization involves compromises.

Besides, we're not talking about a wilderness experience here. We're trying to get dogs and people to live together in healthy ways. Both dogs and people need to learn how to be off-leash in good ways rather than destructive ways.

This means dog owners have things to learn, too. One of the benefits of dog parks has to do with people --- they get to talk to each other and, hopefully, learn how to be better dog owners in the process. The training of humans is an important unwritten mission of all dog parks that we know of.