Part of it is because of lawns, if you have a lot of trees the leaves can simply smother your lawn.
But even in cities there’s reasons to clean them up. They clog drains, they lump together in the gutter creating blockages that cause large puddles when it rains, they stain the pavement, they decompose on the sidewalk and people track it everywhere in buildings, and more. Maybe you don’t see these as problems that need solving, but people aren’t tidying up leaves just for the fun of it, they have reasons. We’re not simply obsessed with cleaning up leaves.
Everywhere in America that I have lived, their primary use case is landscapers blowing leaves and trash from private property into the street, thereby clogging drains.
As an American, I have only ever seen the opposite. Pretty much everyone where I'm at just mows over the leaves. Blowers mostly used to clear streets and sidewalks. Rakes are sometimes used to collect leaves and burn them, but even that is uncommon.
However, I live in the midwest. We don't have as many trees here as the east or west coasts.
I'm on the East coast and I do mulch leaves with the mower for most of my lawn. However there are parts of the property where we have more trees and where leaves accumulate from being blown by the wind from other parts of the lawn. In those parts the lawn would probably die if I didn't clear the leaves myself.
However I also have honeylocust trees which drop loads of these large bean-like seed pods and these stems after the leaves fall which bunch up and can only be best described as like "wads of shaved pubic hair." I also have sweetgum trees which drop these hard, spiky seed pods that I also need to rake up. The mower can turn those into projectiles.
Really the best solution, in my case at least, would be to alter the landscaping in that area to something that would be unaffected by the leaves, but I don't have the time for that now. So clear leaves I do.
You're lucky. On multiple occasions I've had rocks and leaves blown into my face by one of these people, while I'm driving in a convertible car with the top down.
Because someone asked 'why do they do this in America when they don't do it in Europe' and your explanation was 'because if you have lawns trees can smother them'. That doesn't make sense as an explanation to the question unless you think having lawns and trees or not is a difference between Americans and Europeans. Otherwise how does it apply?
All I stated was that if you have a lawn, it is possible for uncollected leaves to kill or damage parts of your lawn.
I don't know why "they" don't do it in Europe, I have no idea where the commenter I replied to is from in Europe. I was just stating one reason why people do it here.
Q: "Why do people in America clean leaves off the lawn?"
A: "Because it's possible it can damage the lawn if you don't in some circumstances."
Simple as that. No idea why people do or do not do it elsewhere.
But even in cities there’s reasons to clean them up. They clog drains, they lump together in the gutter creating blockages that cause large puddles when it rains, they stain the pavement, they decompose on the sidewalk and people track it everywhere in buildings, and more. Maybe you don’t see these as problems that need solving, but people aren’t tidying up leaves just for the fun of it, they have reasons. We’re not simply obsessed with cleaning up leaves.