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
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.