It's not that the politicians are fooled by Keynesian policy, it's that they "pass on" the lie.
Politicians collect rent on the "redistribution" of wealth. They collect kickback, they receive political capital from the people they sucker, and they are able to reward friends and build political alliances by creating new empires.
For a politician to tell the truth would be for him to give up his cut of the policy shakedown.
Associated terms:
"Bootleggers and Baptists", rent-seeking, growth complex