Contrary to common cynicism, it used to be extremely rare for (elected and unelected) officials to actively lie. They'd obfuscate, or refuse, or run away. But outright lies? Almost never.
The problem is: given this belief, and how common it is, why should any politician not start lying, taking bribes, etc? After all, everybody thinks they do so, no matter how they actually behave.
I don't believe this was ever the case. I'm a pretty old guy so tell me what bygone era you are referring to and I'll tell you some of the most famous whoppers, from memory probably.
My opinion is just statistics ultimately. No use complaining about the data. And I'm not a game theorist, but as for higher order effects, I'd bet a systems where no one trusts politicians works out pretty well for all players. The real problem, actually, is that too many people trust the lies, which gives liars an advantage over honest candidates.
The problem is: given this belief, and how common it is, why should any politician not start lying, taking bribes, etc? After all, everybody thinks they do so, no matter how they actually behave.