>Are you arguing those other, less known places are not beautiful? I can't agree with that.
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.
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.
Are you arguing those other, less known places are not beautiful? I can't agree with that.
There are perfectly valid reasons why Florence and Venice are tourist bottlenecks: they are well known because they are historically important cities, featured in famous works of arts -- paintings, literature, etc -- all over the world. So of course I want to see Venice and cities like it: they have well deserved fame. But they are not the only or even the most beautiful cities, they are just the most well known (again, for valid reasons).
The same reasoning can be applied to many "tourist bottlenecks": of course it's reasonable to want to see the Mona Lisa or visit the Eiffel Tower: they are world famous! You learned about them in school! But are they the best or the most beautiful? Nope.
(For the record: I thought Venice was ugly. YMMV)