Why do people deny themselves?

In my (so far) short life, i have seen many people who have great talent at different things, but they deny themselves, and put themselves down because they think they aren't that good. but why? Is it because they want attention? Or could it be that they strive to the perfection they can never reach? I don't know, but I think its very bad for yourself by doing so. I mean, what is the point of having a talent if you are never going to use it. Even the bible says to use your talents wisely ( the parable of the guys with coins). I know theres a point of overconfidence and ego-headedness, but I am sure people can work with their talents with that boundary. If you are smart, don't deny it, but don't boast about it. If you are good at an instrument, let people hear it, but don't dominate everyone else. I think people should use their talents for the benefit of all, working between the boaundaries of overconfidence, or piggybacking people (i.e. helping people too much).
In the past few days, I have heard people say they are dumb, when they are quite smart, and people who say they can't do things which they are good at, and people who are good at things i.e. music, but they don't want to do it.
If everyone denied themselves, the world would be at a standstill. No famous person denied themselves. Some of them went too far, but the fact is they were confident in their abilities. If everyone was confident in their abilities, without going overboard, the world would operate much easier. Everything would operate as it should, like clockwork.
But what I reckon people should do is not to bottle up their talents and abilities, and not to deny them. Just accept it, and use it within the boundaries, for the benefit of themselves, and everyone else.