They tried. Then apple dropped 32bit binaries support.
Apple is a very expensive partner to have. They do whatever they want with their ecosystem and many developers have been burned when apple decides to make their work obsolete or outright copies it and makes part of the bundled in apps.
So. It would be amazing if valve updated every one of their games for new versions of macOS and if they would kept MacOS proton support. But macOS is a moving target that will break backwards compatibility whenever it suits apple. So I understand that is hard to justify the investment.
In the end MacOs and Linux where less than a 1% of the Steam user base. But one is an open ecosystem where there is competition and some semblance of respect for backwards compatibility and the other is a closed and sometimes hostile environment.