This claim right here is one of the problems. They don't all do it all of the time. Can you tell me some of Jeremy Corbyn's lies? Or Zarah Sultana's? Or, for that matter (just for balance) Vince Cable's?
It appears strange that the Tories make so little effort to even appear honest and truthful, but when you look at the bigger picture it actually makes sense. The rational response would be to conclude that the Tories are utterly corrupt and need to be replaced in government, but most people are not very rational, are highly suggestible to the media and are drawn to simple extreme answers, often involving a kind of nihilistic giving up that absolves one of having to keep thinking or trying.
So what they conclude instead is that "they're all the same" (ie, ALL politicians, not just the ones actually lying), and therefore they should either give up on politics altogether and not bother voting, or vote purely cynically in terms of narrow self interest. This actually benefits the Tories because it makes people tend toward small government, laissez-faire policies (ie, Tory ones). If politicians of all stripes can't be trusted, the obvious answer is to elect the ones who will at least stay out of the way and impose on your life the least, right?
It's win win (for the Tories, that is). They get to do whatever the amoral f* they like AND have it blamed on everyone collectively, in a way that makes them come out looking like the best option.
I'd love to be a Martian observing the British public.