Because travel is polluting for a start. When we get cold fusion teleportation with no environmental externalities then everyone can live 100 miles from work if they want. When we don't have to carpet the world in roads and internet cables and have people drive past their house to deliver mail every day they can live as far away as they want. When they aren't burning fossil fuels to heat 40000 square feet of living room then they can live in the biggest house they can build and I won't care. Until all those things are true, I'm going to argue that dense urban living is better and we should be encouraging it or at the very least zoning for it and allowing it.