Their software. I don’t want to go to a site and download a game to find an actual functional launcher (and file management, etc) somewhere else. GOG galaxy is terrible on windows and doesn’t support Linux, despite the overlap between their philosophy and Linux users.
Steam isn’t just a store. It manages my large library with no work on my part, including reasonably high quality tags to make it easier to find games for whatever mood I’m in. It completely seamlessly handles Linux support on almost all of my games, while giving me all the freedom I need to make changes in the rare cases their out of the box setup has issues. It has an exceptionally high quality input mapping tool that is done per game and has a large catalogue of user generated control schemes. It handles simple modding for a lot of games that don’t need anything too crazy. It handles cloud saves so invisibly between devices that I almost never have to think about it.
I will (and have) pay for a game on Steam when I have it on GOG for free, if I actually want to play it. I’ll eventually be self hosting almost all of my other media, and have taken steps in that direction, but I definitely will not be doing so for games. Steam is just too much better than any third party options.
To be fair GoG selling point is that it doesn’t use any external software, it tries to emulate the old disk feel.
Personally I identify much more closely with GoG philosophy, i.e. mostly no DRM, manage the games on my own, etc. However I use Linux, and Steam has been investing into it so I’ll keep giving them my money (the input management is indeed great, but not enough on its own for me).
So what’s the problem about using third party clients like heroic game launcher ? Or did I understand the first line of your post wrong ?
Heroic is just as terrible. None of the alternative ways to manage game libraries support any of the large list of features that Steam does that I rely upon to make PC gaming a comfortable experience, and that list was far from comprehensive.
Until there’s an open game library management tool in any way comparable to Steam, DRM free has no value to me. I’m willing to (and want to, for the things I haven’t yet) self host movies, ebooks, audiobooks, TV shows, etc, because you can get a functional experience with them. I am not willing to do so with games because you cannot.
Steam makes millions of dollars for Valve. They can afford to put a lot of work into making it impossible for anyone to ever catch up to them. If you will never use anything else until it has feature parity with steam as well as having other upsides compared to it, you are never going to benefit from the other upsides.