It is a bit more complicated than that, by the time GOG even enters to the minds of most developers they have likely already spent their entire multiplayer budget
.. so they go on to add support for Galaxy DRM multiplayer? And GOG won't make it DRM-free, even though they could. Come the fuck on.
I said most developers, not all and unfortunately some of them seem to be so used to implementing their multiplayer support through the Steam API that they would rather hack in the Galaxy API to replace that than start from scratch.
Right now the situation is that most
games here that have multiplayer have it DRM-free. A few require CD-keys or third party services. Increasingly, new games only support multiplayer through GOG's DRM.
That of course has nothing to do with releases of old classics with "old school" multiplayer support happen less and less often nowadays while the newer games have their multiplayer being designed for the more numerous modern multiplayer gamers who rely on various online multiplayer features that would not be possible or would not really work well with the playerbase being divided between countless player hosted servers...
Somehow, GOG can strong-arm and pressure developers into removing their DRM and adding all sorts of Galaxy support, so I'm sure they could also strong-arm some of them into removing their DRM from multiplayer (and GOG could remove their own DRM).
Turning a DRM'ed single player game DRM-free only requires cutting out the DRM and in most cases you would not be able to tell if that has been done or not if I would only allow you to access the game from it's main menu whereas some forms of multiplayer would only become "DRM-free" by being replaced with some more DRM-free friendly form of multiplayer, which would take a lot more resources to do and would become an additional thing to support, so we are talking about many orders of magnitude greater expenses than what you seem to be thinking.
The rest of your post is you explaining that some of you don't care about multiplayer, which is fine and I already know that you don't care. The fact that you don't care doesn't make the game DRM-free. I just had to make that point because it puts the DRM of a small part of single player content into perspective with an entire half of the game (or however much multiplayer amounts to; for some people it's the entire game and the single player is just a tutorial) being locked behind DRM.
Actually, my point was that regardless of how much one might care about multiplayer, some forms of it may have the online only disease spread too deep into their design for GOG to have any hope of convincing the developers to do anything about that as once their server requirements jumped to the datacenter category, there may be no easy way of coming back from that to offer any dedicated server support for us.
And if it is not easy, then it might as well be almost impossible for GOG to get any developer to agree to do that in order to be able to sell their game here, if even getting the achievements to be ported to the Galaxy API is in the edge of things developers are willing to do.
This isn't about not caring at all about multiplayer, but acknowledging that over the years the old school multiplayer we would prefer has "evolved" into something else that can't always be reasonably expected to work DRM-free no matter how much we would wish for that, so at that point one may as well see if the single player merits alone would be enough to buy such games, as outright boycotting them would only be a bit more likely to change the multiplayer to our liking than all of GOG's customers refusing to play WoW could eventually convince Blizzard to redesign it into a turn based offline RPG.