Because the designers made it that way?
(Meanwhile, Lore-wise..... it's Magic?. To gain the magic you need the wall. Or..... yeah, you can kind of hear the dragon garbling something out, interspersed with your grunts of pain, clashing swords and armor, big stomping noises as the dragon bangs around, etc. But you'd need to hear it alot to actually get the right pronunciation. And you are kind of distracted by the whole "OMG, dragon trying to RIP OFF MY HEAD!"
Getting it off the wall, where you can actually see how the words are spelled, and pick up the proper way to say it, works alot better.)