Huge pet peeve of mine! I've been wanting to build my own apartments ever since they were first introduced.
I don't live in a "big city", I don't want high rise apartments in a city environment. That's just one more way the devs have forced us to play their way and removed the player creativity from past games.
I live in a small US city that has a variety of different types of apartments, from huge houses converted into multiple 1-bedroom or studio apartments, condominiums that house only 2 home units that are connected but that have several of these units within a complex, typical apartment buildings with multiple floors and balconies and a community pool, large homes that have been converted into duplexes, etc, etc.
They put apartments into another expansion pack (Eco Lifestyle) and you also can't build your own in that pack either!
Faking apartments with the roommate system or mods or how others have said by locking the door is NOT the same thing. You don't get to experience the whole apartment atmosphere from neighbors being loud or knocking on your door complaining about stuff, etc.