Can you explain to me then why virtually no one in academic economics believes in supply side economics?

It's been on the garbage heep of economic thought for 25 years at least. See David Stockman, Reagan's OMB Director's famous interview in the Atlantic Monthly in 1983. The Reaganites gave up on the concept long ago.

"Conservatives are not necessarily stupid, but most stupid people are conservatives." John Stuart Mill America is the only country that went from barbarism to decadence without civilization in between. Oscar Wilde He who dies with the most toys is nonetheless dead.