Question

why do you call them apartments if they are not apart??

Answer 1

They are called apartments, because even though the rooms are relatively close to each other, they are all split (apart)  from each other, and different families occupy different ones (one does not have access to all the rooms unless you own the apartment or work there)

hope this helps

Answer 2

 The word “apartment" comes from the French word appartement and the Italian word appartimento, both of which mean “a separated place."