My assumption was that on Linux it's just been using inotify or something for a while and so hasn't needed a bespoke monitor. I have no idea if that's true or makes sense though.
Of course, in a large enough monorepo Linux performance would also suffer, but to a much lesser degree.
Also, conveniently, both Windows and macOS have an API for recursive directory watch, whereas Linux doesn't (in Vanilla kernel). Inotify can only watch the immediate directory you're observing + there's a pretty low default limit on the number of inotify descriptors that you're allowed to have on top of that
$ time git status
real 0m0.324s
user 0m0.197s
sys 0m0.425s