Because it is and always was a large purchase and you want it to maintain its value.

Quality of your life depends not only on your house but also on houses around you. When I go out, I want to see nice, well maintained houses with no garbage outside. What happens without HOA in low middle class and poor areas are broken cars, garbage everywhere and unkempt lawn. In wealthy neighborhoods you generally do not need HOA as people maintain their property on their own.

Only in newly built out places that didn't have preexisting and sufficiently expansive local governments that could be utilized for such tasks. Those municipalities simply passed local ordinances, many/most of which are still on the books and continue to be used for arbitrary enforcement exactly as envisioned the better part of a century ago.