I've seen a lot of questions regarding why isn't there a guard on lightsabers, but none of them (nor the answers) I've seen tackle the subject of the lightsabers' weak spot (at least in my mind, I may be mistaken).
When two Jedi/Sith are in a duel, often times they not only hit each other, but struggle a few seconds against each other's blade. In those moments, what stops them from sliding their blade to the hilt, destroying their opponent's lightsaber?
The first argument that comes to mind is if they would do so, they would remove their own guard (since they were repelling their opponent's strike) and they would risk being cut down. But I think it would be possible to move out to the side while still sliding down…