why politicians lie

Because they can:
Fact-Checkers Are No Match For Romney and Ryan:

" Asked about the controversy later, Bush's press secretary, Peter Teeley, made a stunning statement. "You can say anything you want during a debate, and 80 million people hear it." And what if reporters then wrote stories demonstrating that the candidate had lied? "So what?" Teeley responded. "'Maybe 200 people read it or 2,000 or 20,000."