Valve focused on Linux because their SteamOS is based on Linux, and macOS competes with it. Mac Steam is Intel-based, not Apple Silicon, yet.
Huh? Steam is a platform for Windows games and was built because MS didn't offer such a thing. Their handheld devices run Linux and a Windows emulator.
I don't think they ever cared about Macs. The Mac gaming market is vanishingly small. Windows gaming is bigger than all other platforms (including consoles) put together.
I used to have Mac Games on a Macintosh SE, and later on a Bondi Blue iMac. This was before Steam came out. There used to be a Mac market for video games, but the Mac is used for creative content and programming mostly.