Just because they don't want to build for and support a relatively tiny platform does not mean that they're hostile toward that platform. They support it in other ways via their game engine. Steam, on the other hand, has a vested interest in supporting Linux due to SteamOS, which they hope to pursue again in the future.
Just because Valve has a lot of goodwill in supporting Linux, even though Proton hasn't really affected Linux's market share in gaming, does not mean everyone else is actively hostile toward it.
Don't forget when calculating profitability that you also have to account for opportunity cost: the devs you hire to work on Linux could instead be working on improving your fledgling client for the much more profitable Windows platform. Valve is already over that hurdle with Steam, so their profitability calculations will be different. You can't draw a direct comparison.
Like I said, I work on Linux. Trust me when I say there's a lot of very good reasons not to support it that have nothing to do with one's personal opinions of the platform, or whatever it is the OP meant by "incredibly hostile."
Meanwhile, Steam is doubling down with Proton. Since Proton, I don't have a Windows partition around anymore, it just works for an enormous number of titles.