I can maybe buy the argument that some foreign names are difficult to pronounce in English and therefore needed to be simplified but I'm driving through Italy right now and almost every town and city is slightly different. Milano to Milan. Genova to Genoa. Firenze to Florence. Yet Pisa is Pisa.
So why were these originally bastardised in English?
This isn't an "English" thing: a quick scan down the "Languages" sidebar of the Firenze Wiki page shows that other languages have many more variations: Florenz, Florencia, Florentia, Florensiya, Florenceje, Firenca, and on and on. It's the same for country names (Italia, Italy, Italien, Italsko...).
This recent post includes some discussion and links to several more posts (including the /r/AskLinguistics FAQ:
Bastardised is a strong and not very helpful word. When foreign place names are frequently spoken in a language, they become naturalised to the patterns of the speaker's native language. This may end up having taken place in the past, from a different form of the current name.
Perhaps a better question more suited to this sub is when did these place names take their current form.