Swords aren't that heavy. My katana is just 1.2kg with the scabbard. It's quite a simple katana, and especially scabbard, but I doubt even the more decorated ones would make a combined weight of more than 1.5kg. It would be inconvenient for a sword to be much heavier because it's basically a long bar of metal you hold from the other end. The force applied to the hand(s) includes torque, so it's different from holding a simple cube of a similar weight, whether you are only hanging your arms down or holding them in a more horizontal position. It's a compromise between effectiveness (as a weapon) and a comfortable weight.