Why is f/1.8 so common when it's not a full stop?

a large proportion of lenses, across manufacturers and time seem to have maximum apertures that are full stops on a standard scale that starts at a theoretical f/1. So f/2.8, f/4, f/5.6 etc.

Now of course there are a massive number of exceptions but I was wondering why f/1.8 is so common as a fast prime rather than just going for f/2.

Sorry if this is a silly question.