A couple of reasons this is true (besides basic human nature):
1. We ascribe more economic power to the president than he or she really has.
2. Our current two-party system forces candidates to slalom out to one extreme for the primary and then back to the center for the general election.
3. The entitlement mentality among people in power, who think it’s perfectly fine to lie in pursuit of their greater goals. Now if only the media could discern what those actual goals actually are.