Sometimes you can. Depends what you want out of it. I've only spent a very short time in London, but I live in New York, and some evenings all I need to do to be happy is walk to the middle of the Brooklyn Bridge and look at the lights and the people. Same reason I don't really need an itinerary if I head upstate - it's enough to find a random mountain, climb it, and look at the trees and lakes. Just getting to know the feeling of a place can be very special.
I am not sure what you mean by that. Do cities not want people to visit them? You can't travel through a city unless you commit to live there for a while?
I’m saying that if you want to enjoy a city you should find things that you enjoy to do there. If you just show up it’s not surprising that you’ll be unimpressed.