It doesn’t make sense for the individual unless you consider that they live in a society. You would be annoyed to have to retrieve a wayward cart and by returning your cart you enforce the expectation and social pressure on others to do the same. I don’t think it’s selfless to treat your environment with respect, it’s in your own best interest most of the time.
I added a new pc to syncthing this week, I installed it and scanned a qr and was done in like three minutes.