Why do people believe in agnosticism?

Agnosticism is just the belief that it is not possible to know with certainty something about a certain subject. The subject it is commonly applied to is Religion and God, but it could be applied to any subject.

As far as I am concerned it is simply a recognition of the finite limits inherent to being human and I consider myself agnostic on certain subjects because I do recognize and admit my own limitations. If I did not consider myself agnostic on those subjects and claimed certain knowledge of them it would be a display of extreme pride and arrogance.