Same question as liudas123; also, if it is for convenience’s sake, the itch.io launcher/app makes it fast to install and play games (although it does not work for all games). And furthermore, when the games are well packaged (and they seem to be, generally speaking), it is quick and easy to download and play them. Except maybe if you have an operating system that is less often targeted.
Viewing post in How many games have you played and rated?
It's mainly convenience sake, I think. That and almost everyone can play on browser (except mobile). I didn't want to leave the lowest rated games at 1-2 rated. I even rated the android one. I'd like the actual download games get the same love, though thankfully the one I was rooting for ended up coming first.