First , reputation matters. If politicians break promises, voters hold it against them. If they do a good job, they reward them. Even if politicians only stay in one office for a few years, they want to build up a good name in order to rise to higher offices. Even when politicians plan on leaving politics entirely, their party rewards them for protecting the party's image.
Politicians can destroy their whole reputation with one mistake.
E. Second , political races are at least as competitive as markets. Incumbent politicians know this, so they strive to preemptively adjust policy to please the electorate. They are the best. That is why they won in the first place and why they are likely to win again. Campaign contributions are just another sign of a serious candidate.