Because there are PLENTY of idiots and suckers out there who buy it EVERY TIME..... again and again and again.... Same for those who keep voting the same way EVERY election hoping for different results. So, are we an Idiocracy yet ?
No because people would rather hear beautiful lies, then ugly truths....Just look at the Trump situation.
To get elected, not every politician lies. However, the truth he (or she) tells offend roughly half the electorate. Offended either due to long held political loyalties, personal prejudices, or just plain ignorance of facts. For what ever reason, the willingness on the part of voters to accept their candidates' lies is what maintains the status quo in our government, and allows money increasingly influence policies detrimental to the public's best interest.
sometimes they lie because they dont know much..prime example was obama in one of his first speeches he said that new businesses would not have to pay capital gains taxes the first year they started...well they dont and never would..he didnt know what capital taxes were..that was a giant red flag for me..and sure enough he was as dumb as he sounded...