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.
well i think people are not honest in some case because they have reasons and they have their own explanation why they do dishonest things. some people are dishonest because they cover up the truth that may create a big misunderstanding, some do it just because they want to tell lies but most of the time dishonest people are the sacrificial people.