I haven't really wondered this, to be honest. I've been more amazed at the number of deaf people who can learn to speak, or- like in Beethoven's case- become so skilled with music. Last Saturday at the theme park I work at was Deaf Awareness day, and I was shocked to see how many people came to the musical show that I do, even when the interpreters had already left. I imagine it was more for the dancing (I highly doubt they can feel the vibrations from our crappy drums, aside from the fact that we're on a raised platform and they're on concrete), especially since one of the deaf girls approached one of our dancers and wanted to learn some of our dance moves. It sucked, because I wound up being interpreter for the day, and all off of only two semesters of Sign Language.
That said, I've studied Sign Language for about a year now, and I've been trying to find deaf communities to learn more/better my sign language, possibly even to become an interpreter, should my stunt work not work out in the future. It's a fascinating language to learn, in my opinion. A hell of a lot more fun than my French and Spanish classes I took too.