Because cities have a lot of different kind of people, different kinds of shops, art spaces, restaurants, performances and so on. Suburbs are far more homogenous. They're like that bar in Blues Brothers that have "both Country and Western".
And cities are a lot more accessible; when you get older you may no longer be able to drive or get around easily, and you will certainly start to appreciate the closeness to various medical specialists, nursing facilities and emergency services.
One major trend here in Japan is that as the population grows older, so does the move into urban centers accelerate, and that's exactly for this reason. Baby boomers are selling their suburban homes and rural houses to get convenient, accessibility-adapted apartments in the city.