> Maybe there is something I'm not taking into account but I have a hard time seeing the meaningful cost of some obscure wiki page merely existing.

The thing you're not taking into account is that every article that exists takes up some amount of editor time, which is why it's good when more people participate in Wikipedia. You are correct that the server/bandwidth cost of almost all articles rounds off to zero. That leaves just the cost in "an actual human looked at this and okayed it," which has different scaling characteristics.