They actually do move... cities/towns/place names expand and contract, and incorporate and unincorporate all the time... it's a huge pain in the ass for our system at work. Some place names are also 100% made up by the USPS and only used by them, and guess what, you still gotta support it. Even the professional paid databases don't contain every possible place name people use.

And my comment was more about geocoding of exact addresses, not whole cities.