Even the best designed city will have different things in different parts of it. I've been all over the planet - from the backwoods hickville I grew up in to beautiful metropolises in Europe to Asian megacities and no matter where I went there was always a reason to travel around. A particularly beautiful park. A temple floating on a lake. The best damn hamburger you'll ever eat. That doesn't change just because I live in a place, and it doesn't mean it's poorly designed. Different people in different parts of the city make those parts of the city different.
I noticed you completely glossed over friends and family living in different places. That's not just some side point. People travel to see other people. I can't force *everyone *I love to live within a 15 minute walk of me - even if I could, they have their own friends and family, and they have theirs, and they all need to work, probably not in the same buildings. If I'm friends with a Construction worker, a tech bro, an RN, a Highschool teacher, a personal trainer and a genetics researcher, are we all going to be able to live in a 6 block radius? Possibly. But it'd be a lot easier for all of use if we could live a little farther apart (maybe near where we each work) and just take a train to hang out somewhere central.
Life sprawls. It doesn't mean rural, it doesn't even mean suburbs (even though that's how people seem to want it today). But it does mean transportation is a requirement.