This doesn't represent all computers, just those that have Steam installed. And since most games on Steam are for Windows it makes sense that it be biased in that direction.
But the point isn't to represent all computers: there are hundreds of millions of machines of all OSs that will never have Steam installed on them.
The point is what is the breakdown of Steam users by their platform of choice. Even when there is a somewhat substantial macOS-compatible catalog, 95% of the users don't run Steam on macOS. A lot of that is due to lack of developer interest in publishing for Mac. But, on the same hand, the lack of developer interest stems from the lack of support from Apple for supporting a wide range of graphical APIs (removing OpenGL was the death knell - no major developer is going to spend all the time porting to Metal for a measly percentage of sales).