That and mature trees.
That and they WANT to be able to tell their neighbors when to mow and what color they can paint their house (as I'm sure well see from the posts defending HOAs that may follow).
I live in a non-HOA area and yes, we have a couple neighbors that don't take care for their own property as well as I'd like but that's just it it's THEIR property. However for the most part our neighbors do just fine without all the nannystaters and busybodies (and most of the ones that don't are old widows ... who wants to hassle old widows on a fixed income? Authoritarians do, that's who ... a pox on them).