Because there are lot of people who are single/couple, living with boy/girl or just friends, going to university, traveling job and so on. As you can see there

Because there are lot of people who are single/couple, living with boy/girl or just friends, going to university, traveling job and so on. As you can see there is large category of people who are not raising families or planning for long term in city. So obviously there is big demand of apartments but not family size apartments.