I like what Bob said, a lot of it is from people being scared to try new things, either because they think it's weird or they might be "tricked into something." I say "Pah!" to them! :P
Something else I've noticed though, people really do feel they don't "need" the self help. They resist the thought because they believes it means they are broken and this will fix them, and noone would like to think of themselves as broken. As I've offered books for others to read, almost always a book that would be valuable to them, I often get a response of "Nah, I'm fine as I am." Mostly it's the ones who are proud that do this, as those that are humble have already overcome that kind of thinking.
I would say this kind of resistance to improvement is driven by the egos need to feel superiour. The ego in us could not bear the thought of not being the best, and the best can't get better so self improvement is a threat to the ego's view. When you use this to look at how people view self development, it makes a lot of sense.
Also Steve's post on the topic is a very good answer for the question.
Self Acceptance vs Personal Growth