I mean for a time they weren't, which is why germany (and berlin itself) was partitioned into western and eastern germany under the administration of various western countries and the soviet union in the east

/u/abrytan wrote an great comment on why germany was not partitioned after ww1 and touches a bit on ww2

https://www.reddit.com/r/AskHistorians/comments/7vy6fv/if_german_unification_in_1871_threw_many_of/