That is because Japanese Kanji system is quite complicated, and many names of places have kanji with irregular pronunciation, strictly speaking you cannot be sure that your pronunciation is correct if you never hear the name before. (you can make a very good guess, though.) Japanese Wikipedia even has a list for the places with name not easy to pronounce correctly. (難読地名).
Lots of names, especially geographic names are morphed in other languages.
Paris is pronounced "pahree" in its own language, "perris" in English, the we Dutchies manage to get "pahreis" out of it.
Same goes for Japanese cities, although double vowels are often omitted in Romaji as far as I can see.
“Nobody wants a house in Osaka,' he said, and it was strange to hear him switch suddenly to foreign pronunciation in the middle of his English. 'It would mean you had to live in Osaka.'
'What's wrong with it?'
'It's like . . . Birmingham.”
-Natasha Pulley, The Watchmaker of Filigree Street