They were talking about exactly this on a news prog the other night. Because politicians are always vying for votes (new or to retain) we get all the guff, the rhetoric, the desire not to directly answer questions, the pivots, the misdirection etc.
Politicians and their parties promise the proverbial earth, get into power and deliver 10% of the promises. Another party gets in, rinse and repeat.
Mind you, when they do try to tell it as it is (didn't Labour sort of attempt this early in their current term? It'll get tougher before it gets easier) they often get feedback from the media and public for being too negative!