Could be any number of reasons, but I bet fear is a big one.
Fear of ridicule
Fear of being judged or getting into trouble.
Don't see the point in hurting someone over a mistake.
Don't see the point in hurting someone over something small.
Struggle to express themselves.
Lots of reasons I guess.
I am very honest. Probably the reason I have very few friends. It really doesn't matter to me how many friends I have. What matters to me is that they know me for my integrity. People know me as a honest person not as a liar. This means more to me. This is who I am and I love myself for being honest.
Most likely for the same reasons you are not sometimes.
To quote a cliche "You can't handle the truth." Most people lie to protect the feelings of the other person, and the reaction the true answer will get them. Lies generally leave the receiver feeling rejected and therefore non-combative. Gives both the chance to walk away without being real.
This is funny. I'm too honest, and it gets me into trouble.
The other way is easier.