Nurse Joy and Officer Jenny were characters added, in my opinion, because Pokémon is a kids series and it made things easier.
Think about it, if you're a kid then you won't really look to learn different types of names just for one nurse because she was a minimal character. I know if they had different names for her, I would have made it easier for myself and just called her 'nurse'. So, to have one name for every nurse in the game just makes it easier for people to remember and note. The same goes for Officer Jenny.