Why are apartments called apartments when they're stuck together?

...thanks for frying my brain

Wow, I haven't thought about that before in my life.
But now when I think about it, it's a very good question.
I really want to know the answer.

It's just like "Why is it called a building when it's already built?"

If I was high, I would have probably shot myself by now.

But since I'm drunk, I'll say "FUCK IT", and have another drink.