There isn't just one well-defined crack. Where the plates are separating there is a whole zone of material that is being pulled apart and cracking open. The current crack is just one of many many fissures in the rift zone. And the individual fissures are jagged and ziggy-zaggy, so even if they had clean sidewalls you still couldn't see straight down to the bottom.

Also, the crack is not filled in all the way to (near) the top. There are gaps and voids all the way down. The "bottom" that you see is just the first filled-in part. If you dug down you'd probably find more gaps. (I'm not 100% sure about this. I know about the mechanics of slip-strike faults because I live in California within walking distance of the San Andreas, but the African rift is different so I could be wrong about some of these details.)