Selling managed software is definitely a hard problem that game studios don’t have expertise in. It’s cool that the community reverse engineered Quake’s server, but that doesn’t mean it’s trivial for all studios to offer server software for their titles.
Implementation aside, I think it’s primarily a money thing—the studios don’t care about this because the numbers say they shouldn’t.
Even peer-to-peer multiplayer was very common. That is sufficient server software was included in the game itself. No need to download or run it on a server.
Online-matchmaking becoming more and more centralized really made this starting to go away... It has both good and bad sides. One is much larger player pool and automated matchmaking. Other side is losing communities and more custom servers with custom maps...
Implementation aside, I think it’s primarily a money thing—the studios don’t care about this because the numbers say they shouldn’t.