Well, let me insert my two cents...
I'm a drummer. I'm also a Music Composition major at the University of North Texas. The reason, I think, that there are so many bad drummers is because drums are probably the easiest instrument to pick up and play without any knowledge. Think about it - when you go to Guitar Center, everybody walks through the drum room and just taps on stuff with a stick. With a couple minutes of practice just to get coordinated, you can play a basic rock beat (and the entire AC/DC repertoire...) But with guitar and other instruments, you have the whole concept of pitch added, so you can't just tap a guitar and make a "decent" sound. You have to have a basic understanding of some chords and progressions and scales. So overall, the reason there are so many bad drummers is because they need the least amount of musical knowledge in order to "get by".
However, I've known and played with way too many bad guitarists, bassists, vocalists, etc. I mean, how many guitarists learn from tabs and never know anything about chord progressions, scales, structure or anything? I've been in bands where I, the drummer, know more about what the other guys are playing than they do.
The worst musicians of all, in my opinion, are vocalists. Everybody knows how to talk, and with some practice, you can develop the ability to stay on pitch. But they have no concept of any music theory at all. I guarantee that in your high school choir, at least 30% of the people don't know how to read music and probably over 70% don't know how to sight sing. They just listen to what they are supposed to sing and memorize it, so they never learn how to read music, much less anything about music. At school, the worst musicians in every one of my theory classes are always the vocalists. Oh well.
I'm not trying to get all high and mighty here, so hopefully nobody thinks that. In a nutshell, I'm just trying to say that there are plenty of drummers that are great musicians who know theory and everything. But there are a lot of drummers who don't know anything about any of that because it's such an easy instrument to just pick up and play. Hopefully this all makes sense to you guys.
--Ryan