They didn't use bullets in the movies. The mechanics of blocking a bullet with a lightsaber are somewhat less forgiving than those of blocking a blaster bolt, and even if they manage it, they've still got a spray of molten mist to deal with.

And like I said, even if it didn't kill them (and it usually took a lot more than one bullet to get the job done), it broke their focus on their prep and forced them to devote their energies to a defense instead of their attack.