That it doesn't exist on macOS?
The point I was making is that the overwhelming number of games on steam are windows only games, the valve semi-supports on linux via proton, which they do not offer on macOS. The net result is anyone jumping through hoops on macOS to use steam+wine is recorded as a windows user, even if there were even remotely meaningful number of such users.
Hence linux steam users matching macOS steam users is an obviously plausible outcome.
Hoping to see more movement on Mac compatibility in the future, but even with the recent game porting tools, I won't be holding my breath. It's sad, these laptops seem like they'd be decent for casual gaming (One of my main computers is a MBP. I use all the OSes.)