As well as technical issues about how and what code you could hand over, there are likely to be issues around the IP of both the front and backend code, but also of whatever franchise the game is part of. If it’s a good, or at least milkable, franchise concept, they’ll likely need to sell it to recoup sone of their costs. Then, any code they bought in, either as a pre-existing package, or as a subcontracted unit, would be unlikely to have the sort of licence that lets them redistibute it, which means the code will likely be missing key components even if they do distribute it.

Building a basic server that does enough for a small group of players to enjoy is likely to be the simplest approach as they can control what IP goes into the server code, and by not changing the client, they avoid difficulties in that area.