Why every time tax rates are cut does the economy grow and tax revenues increase?

This happened under,
Coolidge
Kennedy
Reagan
Bush

Why was Kennedy the last Democrat to advocate tax cuts?

Tax cuts increase the incentive to make money by increasing the return on investment. If a person invests the time working, the return for that time should outweigh the return for not working, i.e. welfare, other gov't handouts, & even up to other investments. After JFK (even a little before), Democrats became more invested in increasing their power by making people dependent on gov't through socialism. Therefore, they increase their power by keeping taxes high (making the ROI too low to outweigh working) and by increasing gov't handouts.

They all left more money in the hands of people. They spent or invested most of it so the economy grows.
Kennedy could count.

I don't think he was the liberal he said he would be while campaigning.

Remember?
"Ask not what your country can do for you, ask what you can do for your country"
Because they lied and collected more tax dollars.
At the Federal level, there is now enough history from the last couple of decades to show that pure tax cutting is detrimental to the economy and job growth. Ronald Reagan (GOP) cut taxes, but raised taxes shortly thereafter. George Bush Sr. (GOP) had to raise taxes despite a pledge not to do so. Clinton cut some taxes but raised taxes more on the richest. Clinton's actions were followed by the longest economic boom in U.S. history, contrary to GOP doomsayers' predictions.
First of all, it's not every time. Cutting taxes to increase tax revenue sounds illogical, but in reality the more money a company invests the better it can do, and the better it does the more taxes it pays in total. This cycle continues with companies investing even more into themselves to do even better. A strong economy is key for this to work, plus there is some luck involved; you have to have faith that the people will put their money to better use to increase tax revenues.
People invested to make more money and that created jobs to get more tax money. It's how it works. Peace