Sadly, it pretty much boils down to the fact that most voters are ill-educated, stupid, and childish. It a politician tells a voter what he wants to hear, the voter likes the politician; if the politician says something the voter doesn't like, the voter simply discounts it and decides the politician is no good. It never even crosses most voters' minds to fact-check the politician, if he likes what the politician said, regardless of how preposterous the statement, the voter is happy to take it at face value; if the voter doesn't like what one politician said, regardless of how true the statement may be, but another politician says the opposite, the voter is happy to go with the second guy.
There may actually be upright, incorruptible people who wold never lie or deceive, or practice dishonesty under any circumstances, and who would be delighted to serve their community or nation in political office, but it's clear how the process would quickly weed them out, leaving only those who basically have the characters of con artists, swindlers, and used car salesmen. Look, for example, at Rick Scott, with multiple felony convictions for ripping off Medicare, who used his ill-gotten billions to get himself elected Governor of Florida. Do a Google on him to see how he's ripping off Floridians by lowering taxes on rich guys (like himself) and making up the shortfall by increasing taxes on, and cutting social services for, the common people.
Consider, too, that in order to have any chance of winning, a politician has to have his name in front of the voters, i.e., have money for campaign advertisements. When you consider who will, and won't, receive campaign donations from those who can afford to donate big, and what those big donors expect in exchange, it should be self-evident how this consideration accelerates the winnowing process.
To be sure, there are occasionally a few earnest people, eager to serve the public good, who manage to make it into national political office, like Representatives Anthony Weiner and Alan Grayson (he was elected from Florida for one term and then defeated in the following election). But these tend to be uncommon, often short-lived, flukes.
Any further questions on the topic?
"Anthony Weiner and Alan Grayson "
In picking those two you now might now want to mention how you American voters are also too stupid to recognize your own ignorance.
They pick the best they are offered.