They don’t target population centers, they target military bases that always happen to be near a population center. In the 50’s and 60’s missile targetting was shit so they had megaton yield bombs to make sure they got the bases. Nowadays they have lower yield bombs so they can have more bombs that specifically target bases.
It’s no use targeting population centers as those bombs could be used instead to cripple them militarily. It’s just that no country would have a good day if nukes went off anywhere near them.