Well people are not honest because they have personal interest and motive and in order to get it they need to be dishonest or anything whatever it required.
Why people not honest? Some people are not honest because of their motive to cheat people and betray...but those people wasnt success their motive and intension because they are doing bad things,some people are not honest because they have own reason why they should do that and this why they must lie and not to be honest.