Because that would make for a terrible sword fight scene.
It is interesting to think how you would do battle though, it would basically be the first person / fastest to swing, likely killing both parties anyway.
If there's something lightsabers don't cut through - I think cortosis was mentioned in the KOTOR games - then it would make sense to have a small lightsabre on the tip of a staff of cortosis. (Or perhaps, more practically, a staff coated with cortosis, I don't know how heavy that stuff's meant to be.)
In any case! ^^; A light-spear. You'd be able to parry with the shaft and stab/slice the other person from beyond their reach.
It's not canon, but I remember reading one the expanded universe books that describes a Sith that had somehow created a lightsaber with an extended blade for some perceived advantages. I believe it added 12 to 24 inches to the overall length.
In any case! ^^; A light-spear. You'd be able to parry with the shaft and stab/slice the other person from beyond their reach.