If information is available but costly, a natural way to align incentives is random monitoring combined with over-punishment . You only check in on your secretary once per day, but you fire her if you catch her playing Tetris. This risk might entirely keep her from playing video games at work because she doesn't know when you'll show up. We can easily extend principal-agent thinking to politics. The voters are principals - they want politicians to do a good job, keep their promises, etc.
Politicians are the agents with their own agenda.
One answer is that politicians shamelessly and repeatedly violate voter trust because voters don't pay attention to what politicians do anyway. This story may be largely true, but it does not necessarily follow. Even if voters are poorly informed, they could still induce good behavior by over-punishing . While this sounds unrealistic, for offenses voters care about, it happens. Voters really hate racist politicians, so they ruin the career of any politician who lets the wrong word slip out.