Something of a self-reinforcing statistic. Steam is rarely installed on Mac because there’s hardly any point doing so.
Proud 2.2%er
But I also have had it installed via Crossover at some points to check out a Windows-only game.
Which I just realize also skews the statistic because Crossover basically creates a Windows VM.
Macs have a single digit market share globally, so even if people started installing games on them the needle wouldn't move much.