It's heavy (or heavier than a cassette) because it contains the freewheel mechanism. Many years ago, Campag made freewheels with alloy sprockets; they were ridiculously expensive and rare as hen's teeth. Good luck with finding one. And they didn't last long, obviously.
Actually, I think the combined weight of a hub and freewheel is probably about the same as a freehub and cassette.
It's heavy (or heavier than a cassette) because it contains the freewheel mechanism. Many years ago, Campag made freewheels with alloy sprockets; they were ridiculously expensive and rare as hen's teeth. Good luck with finding one. And they didn't last long, obviously.
Actually, I think the combined weight of a hub and freewheel is probably about the same as a freehub and cassette.