I can't speak for everyone (and are probably an exception, since even for a 3 week vacation my luggage never exceeds 15kg). But I can easily go for a very long weekend (up to a week) with the contents of a small duffle bag that easily fits under the seat.
Which means a fresh t-shirt, underwear and socks for the entire time. It includes toileteries, something to read and a medium sized camera.
In my experience people usually pack far too much.