As a teacher, I love using analogies to help my students understand complex material: life is like a box of chocolates, our blood circulates like a river system, etc.
To me, the dog park is analogous to a gym membership: a good idea in theory, but incredibly pricey if you never go.
We've never joined. I've never joined a gym either. I know myself too well.
To get to the dog park, we'd have to drive several miles. A dog park would defeat my own (extremely modest) exercise goals. The dog park has had a vaccine-resistant kennel cough outbreak. Etc.
Frankly, the main reason we haven't joined is that I don't entirely trust other dog owners.
This website has some wonderful analogies on dog park etiquette. Basically, if you would find a child's playground behavior intolerable, don't let your dog do the equivalent in the dog park.