I always figured the existing world governments still exist in Star Trek, they just operate on a level more like States. The UFP essentially is a bit like the EU parliament: all the countries in the union have got representatives there, but that doesn't mean they don't also have their own self governing powers as well. The UK is still the UK, the USA is still the USA, and so on. It's just that the boundries have changed.
Alternatively: it's an altered universe in the new movies. No reason to assume that the old school boundries haven't reasserted themselves following Nero's intervention.