Because Linux still makes up a small % of PC Gamers, so CDPR hasn't prioritized it. Plus they'd need to have some kind of proton-like middleware (or just proton) for the majority of their games (which are mostly 15-20+ years old) to be playable. It seems like a large engineering challenge for a company which isn't nearly as wealthy as valve
Well it's not going to be the same engineering challenge as it was for Valve, because they only need to integrate proton, not develop it. If proton works on Lutris (via umu), an open source project with no corporate backing as far as I'm aware, surely CDPR can at least attempt it. This is probably the best time to do it, too. SteamOS has been well received and is likely to end up on even more handhelds, and Windows 10 is nearing its EoL. If GoG is one of the first storefronts to allow its users to play outside of windows it might generate a lot of positive sentiment in the community, just like they did with their anti-DRM stance.
Valve isn't public, but they seem to be making plenty off of WINE. In fact, companies of all types love building on other projects, because it reduces how much work they need to do.
They just don't seem to care. They could literally hire someone who works on Heroic to make an official Galaxy port reusing most of Heroic's functionality. Yet they don't.