Such games are often not peer to peer, like the games of old. Releasing a server in a form that's somehow operable by a third party is not always easy.
If there's enough people interested in the game, someone will figure out how to run a private server. People are so persistent it happens even without the source code. For many online games the source code is either stolen or dedicated people black box reverse engineer it just to run private servers. That's how runescape private servers operated back in the day, although I don't know if it was a clean room reverse engineer.. someone probably stole the code given Jagex.
which is why there should be a law in place to force game companies to add LAN even if it is completely hidden from day 1 which should not be usable if companies care about competitive edge but at EOL they should be forced to add a patch that activates this feature