They’ve made Linux gaming a thing. And there where no players there either. So I do not believe that is their actual reason.
The actual reason is that Valve does not want to be beholden or locked to a corporate entity like Apple or Microsoft. They would be very dependent on the whims of those companies.
Linux gives them a platform where they know and can influence it’s future.
Valve Says Counter-Strike 2 for macOS Not Happening Because There Aren't Enough Players on Mac to Justify It (www.macrumors.com)
https://www.macrumors.com/2023/10/10/valve-confirms-counter-strike-2-no-macos/