Because if they could be upgraded to equivalence with a house, with access to features tied to houses - gardening, workshop - then I suspect few people would bother with houses. I know I would switch simply because it would mean I could drop my subscription once in a while if I wanted. As it currently stands, I subscribe regardless of how much I'm playing because I don't want to lose my house by mistake.

I kinda think the devs would be reluctant to abandon all the time and resources put into the houses in the wards which is what upgradeable apartments would probably lead to. Of course that's just imo

Edit: this was meant to reply to Hyomin's question about apartments needing to be demolished, on the previous page, but I neglected to hit reply. I am a very bad person!