It's pretty much essential to the standard D&D plot set up that villages have few, if any, people with class levels. If they did, they could deal with the monster infestation themselves, and wouldn't need to pool their life savings to hire the first group of adventurers who happen along.
Well, that's precisely my point. You are looking for logical consistency, and when you start citing supposed 1e AD&D demographics as the basis of logical consistency, things fall apart very quickly.
For example, if it is essential that villages have few if any people with class levels, consider the most iconic village in the game, Hommlet from T1: The Village of Hommlet.
It addition to the PCs it contains a 6th level cleric, 8th level M-U, 6th level fighter, 4th level thief, and 7th level Druid. Each of those characters has a backstory that gives them the motivation and means to "adventure", as well as largely compatible goals and a common purpose. They are vastly more capable group of "adventurers" than the 1st level PC's the module is intended for, and they have at least as much reason if not more to want to protect the village from the dangers of the moat house. They could indeed deal with the monster infestation themselves, and claim the treasure for themselves.
So why don't they? Because this the set up to a game.
Nor is there any sign that there are 10's of thousands of nameless 0 level characters hiding some where in the setting. Pretty much every man, woman, and child in the village is detailed. And those aren't remotely the only leveled PC class individuals in the little village. Nor do the entries for pilgrims, bandits, pirates, or beserkers suggest level NPCs are rare.
Why are they not as rare in practice as you'd expect in theory? Because this is the set up to a game. In particular, my suspicion is that the Village of Hommlet is filled with potent PC classed NPCs to dissuade the players from looting and pillaging the Village, or to at least make it a considerable challenge for the PCs to do so should they decide to do so.