I live in an urban part of my city, and I still have plenty of reasons to leave my 15-minute walkable area. A friend far to the north, a friend on the other side of the river, family on the outskirts. Restaurants I love all over. My office downtown. Theatres near the university. Festivals and expos spread out across all the parks and venues and other walkable areas. I can use my bike to access pretty well all of those things in the warmer months but transit (and sometimes, unfortunately, a car) keeps it accessible year round.
If you don't see any reason to use transportation in a city, you either don't appreciate the breadth of experiences that come with city life, or you live in a really boring city.