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.

Why would it be any different elsewhere? Plants generally work the same no matter which country you're in.