AFAIK they get a higher percentage per unit sold after a certain threshold of either units or $ had moved.

DRM-Free and Achievements aren't mutually exclusive. The problem is Galaxy adds onto the developer workload rather than reduces it. If DRM-Free stores were smart they'd have collectively advertised to publishers from the start the huge benefit that developers only need one single DRM-Free build that could be shipped to all stores simultaneously (GOG, Humble, itch, etc) without needing to be rebuilt for each store each time, making selling on DRM-Free stores actually more desirable. Instead the "need" for DRM-Free clients like Galaxy messes that logistical simplicity up and here we are today with GOG wanting special integration work just for Galaxy and many devs clearly not wanting the same...

Likewise, GOG's new method of trying to reuse Steam Achievements in 2019-2020 AAA releases via "wrappers" may "cure" this but does so simply by replacing one problem with another (noticeably increased startup times via the over-convoluted method of having one fake Galaxy client try and intercept calls from a fake Steam client both running at the same time even in offline installers). There is just no polite way of saying that "needing" Steam style Achievements in GOG releases has always screwed something up for someone, and whilst it hasn't prevented DRM-Free releases it certainly hasn't been without compromise either...

I don't think Steam forces exclusive deals, otherwise there would be no games that cross over to other platforms. Most developers/publishers simply favor Steam due to its large user base. The path of least resistance is to focus on Steam without having to create alternative builds for other websites.

Problem is, last I heard the vast majority of GOG customers use Galaxy. People in 2020 prefer the client experience, even if they buy on GOG to nominally enjoy the notion of DRM free gaming. GOG is making a wise business decision to focus on Galaxy, despite the endless grief they get on the forum for it.

I say this as someone who never uses it.

I tried Galaxy months after it was introduced. It slowed down my PC considerably, especially when booting up. It even caused one my games to crash. I stayed away from Galaxy ever since. Cloud saves and virtual trophies are not worth the headache.

I agree as someone who used it since the first closed pre-beta test... The question is if people actually expect the "client experience", or if they just use the client for convenience and could live without it. I suspect the former - it's no accident Galaxy (while still beta) was released publicly alongside The Witcher 3. For brand new releases which still get updates within weeks or even days it's - let's face it - PITA to keep track of updates manually. A client for auto-updates is far more reasonable when updates are frequent.
And once you have a client for that, it's of course not the end... people expect achievements, cloud saves, social features too. And the problem is MP where it's the game devs who expect having the infrastructure readily laid out before them.