There's also noise and odor/smoke pollution. An HOA is pretty much the only mechanism to ensure that a group of people who don't smoke marijuana can band together to live in marijuana-smoke-free air. (Yes, some people smoke enough potent skunkweed for the stink to carry multiple houses away. No, the police don't care.)
Most of the things HOAs care about are indeed idiotic, though. IMHO, the HOA's right to dictate what I'm doing ends where my property begins. The point of the HOA is to control/constrain the externalities you have on other houses/communal properties. If you put a garden gnome on your lawn, there's no externality.