I get that on Steam, MacOs was more popular than Linux due to the sheer size of its user base, but how on earth do you play games on a Mac? I got my first MacBook from work AND because it was a work laptop not intended for gaming, but that didn't stop me from installing steam and try... Like a 10% of my Steam library? What?!?! Yeah, I can play Team Fortress 2 and Stardew Valley, maybe some RetroArch for slow working days, but not much else! How was MacOs the second biggest platform on Steam with such a small compatibility list?!
It's because Apple keeps shooting itself in the foot gaming wise. Instead of embracing open standards they made Metal. Instead of keeping Open GL up to date they let it languish for ages until finally Metal came out. Then the transition to Apple Silicon and dropping all 32 bit support killed a ton of old apps.
About 10 years ago Mac gaming was far better than it is today. Pretty much every game that ran on Linux ran on Mac.
Linux took Macs spot less because it's gotten so much better, and more because Apple has shot itself in the foot. Without the steam deck Linux would have overtaken them eventually but mostly because they're bleeding gaming market share.
It's Metal instead of Vulkan. But I've seen somebody else make this argument before and somebody corrected them by pointing out that Metal dates back to early iPhones and predates Vulkan.
For me (15-ish years ago), it was because I only had 1 laptop to take notes on in college, and it happened to be a Macbook. My gaming was limited to blizzard games and anything that just happened to run on Mac in steam, like valve games.
I bet most Mac gamers are in the same boat. They didn't buy it specifically to game, but they'll still play what they can.