The lightsaber is presumably not a very heavy weapon. It's implied to be little more than a handle with a weightless plasma blade, very easy to swing and throw around.
So why do so many people wielding one do so with two hands?
It can't be the weight. And you'd have thought they could do something useful with that free hand. Balance better. Swing from things or punch things during the energetic routines that seem to constitute lightsaber combat. Wield a blaster or, hell, even another lightsaber: we know that's possible because General Grevious does so, as do characters in KOTR (no idea how canon that is).