It adds costs if you built it that way. I don't think many games are built that way. Developers need to be able to test their games in isolation, and it takes effort to remove that scaffolding from release versions (so people can't use it and bypass your monetization).

The real reasons to not just toss your backend over to the community and make it their problem are business reasons like 'it will dilute our brand' or 'it is a violation of licensed IP'. Or embarrassing reasons like 'we have lost the source code' or 'we can no longer build new executables'.