Why do tax brackets work this way why is it that a Rich/Wealthy person the first few 000 dollars may be taxed?

Not sure how to answer this. I do know that when FDR taxed the rich at 91% America was and continued to be the strongest wealthies country in the world and the rich were still happy, we had a strong middleclass and a very sound economy. When Reagan dropped the tax rate on the rich and the corporations and lowered tarriffs to suicidal levels our economy bumped up slightly not because of this but other reasons then began its 34decade long downward spiral to the near demise of America we are now witnessing. 2% Tarriffs are suicidal japan and china have tarriffs of 30 and 40% because they care about there economy and have yet to succumb to corporate greed.