Of course they do. The better question is “Why do politicians lie to us?”
In the United States, we have two political parties. Yes, there are other parties than just Democrats and Republicans, but how many of those other parties actually matter? Therefore, successful candidates for office, at almost every level, are either a Democrat or a Republican. These parties are composed of huge numbers of individuals with similar political beliefs and principles.
As in any large organization, political parties are led by a small number of highly motivated and dedicated individuals who organize the membership and set the agenda.