Along with "lack of affordable housing" there are plenty of motivations. A big factor is desire for quality of life. One's commute, social life, and networking capabilities are all enhanced with a rented (expensive) home in the city.

I'm under 30 years old and doing well in my field for my age. I've lived in two major US cities so far in my professional life. Of all of my coworkers and people I've met my age or younger, I know of only ONE person who actually owns a home--and he and his girlfriend got a big gift from her parents in order to put a down payment on it.

Everybody else is renting apartments in expensive downtown or "hip"/"young" neighborhoods that are only becoming more and more costly. None of them live in the suburbs. A handful of them don't own cars, never have, and never intend to. They all walk, take public transit, or bike to work.

I personally don't intend to own a house anytime soon. I don't intend to buy a car anytime soon. I don't think I'll ever have children--and at this rate I certainly could never afford them. I'm not alone in these sentiments among my peers.