Good, and tough questions. I don't have a lot of experience with Karate per se, so others may be able to speak better to that. I've got a whole bunch of writing I've been working on w/r/t to this topic...I'd love to see other people's input.
That said, the average martial arts school advertises something like:
Whether they really offer all of these things is something I'm quite skeptical about.
I would guess (and what I suspect you're driving at) that many students start out in Karate to learn to defend themselves (or say they do). Whether they ever get that is a big question, and I suspect that a reasonable percentage of those who leave do so because they feel their training has not really served them in that manner.
Of course, they can drop out for other reasons a well. Sometimes people's goals just honestly change. Life happens. Martial artists are a little odd in that we're one of the few branches of physical culture where we expect people to keep doing the same thing for their entire lives. When a kid stops playing baseball, or decides to take up Olympic weightlifting instead of powerlifting, no one is particularly shocked. When someone decides they don't want to do martial arts any more, we act like it's a big shock or oddity.