Yes, they have to. Which says a lot (negative) about the American system.
They lie for three basic reasons, which are all interrelated:
1) they lie because they have multiple constituencies and they want to appeal to (and get the votes) to be elected and reelected. This means, among other things, that they tell each group what that group wants to hear. Which means that some portion of the time they are lying.
2) same general answer, but more related to funding than electability, although they are tightly coupled. You don’t get elected without money, and the more someone contributes the more you are likely to ‘bend’ your thoughts to the folks who pay the bills.
3) Politicians are narcissists. They have a hugely inflated sense of self and sense of importance, which tend to outweigh any degree of honesty or consistency. Basically, the truth is what they say it is at the time they say it.
Together, there all sorts of incentives to be dishonest, and no incentive to be honest.