What are you in about, mate? Of course they "don't have a problem" with being in Steam—they have almost all the marketshare!
Humble is a Steam key reseller.
GOG is a completely different, DRM-free storefront.
Sure, Humble has limited marketshare (just like GOG), but with Humble, they just send a batch of keys and wait for money to come back. With GOG, they'd need to support an entirely different storefront for, potentially, the same limited revenue potential they'd get from Humble.
Not at all comparable. And totally reasonable that some publishers/indie devs aren't willing to put their games on GOG. Disappointing, of course, but not hardly surprising.