Why is the third person point of view unreliable?

You never know if they are altering the truth or simply not telling the truth at all for one reason or another weather it be there personality, to hide something, or to make themselves look better.

First-person narrators are often unreliable, because they often have an ax to grind, they're children, they're handicapped somehow, or for other reasons. The convention--the normal assumption--is that a third-person narrator is reliable, unless the text contains evidence that the narrator is not reliable. In fact it's hard to think of an unreliable third-person narrator.