Yeah it's also that people can't really do grand tours any more so to get the most tourist bang for their tourist dollar people are naturally going to go for the most important/historic/beautiful sights. Up to a point it's probably also self reinforcing when everyone is going to one place it's has to be special so more people go there.
Well, yes. Milan or Napoli or Brindisi or whatever, is no Florence or Venice or Rome. And there are nice cities like Verona that are nonetheless not in the league of the latter.
You can find nice landscapes all around Italy, but if you want to visit cities, some are naturally more beautiful than others.
Like how Santa Fe is nicer looking than Oklahoma City.
>But they are not the only or even the most beautiful cities, they are just the most well known (again, for valid reasons).
Well, they are the most well known for their beauty. Part of it is totally objective reasons: they were huge wealthy patrons of the arts (Florence) and capitals of naval empires (Venice), so they have more major works (architectural, etc) built than others.