> given that an HOA is completely unnecessary...

This is true sometimes, but if you are upset at a specific HOA, look at your docs to see what it actually manages. HOAs exist to pay for community obligated costs; sometimes that is a pool, sometimes it's "water and gas bills", and sometimes it's an entire golf course.

Further, if you're talking about a condo building (which, yes, is technically a COA, but the two get combined all the time), it is literally paying for your roof and elevators to work.

Buying into a "little club" and agreeing to pay your share of expenses only works when everyone participates. If not, your neighbors are just paying upkeep on your property; why should they have to do that?

Conceptually, HOAs seem to be similar to medieval guilds. They are private governments, where voting rights are based on a private right (property ownership) rather than involvement in the area the government controls (residency).

Such arrangements used to be common, but they have become increasingly rare in the past few centuries. Many were explicitly abolished by law when free markets started getting popular. Often for antitrust reasons, or because people thought it would not be in the public interest to have certain types of contracts the previous owner made also bind the new owner.

I can think of a few similar arrangements in Finland. For example, membership in student unions at research universities is still mandatory, as is membership in reindeer husbandry associations. Those entities perform government functions, and the actual government has chosen not to assume the responsibility for one reason or another. As a consequence, the entities and their officials are subject to the same regulations as the government when wielding powers used to justify mandatory membership.

Condos are another story. If the entire complex is a single property for legal or practical purposes, it makes sense to have a single entity in charge of it.