The Laffer scheme ignores the influence of human nature. Keynes understood that humans will always default to their basic values and behaviors. People who invest in job creation have a generally shared behavior. They do not invest their own money. So, if you give them a tax cut, there is no guarantee it will result in investments that create jobs. Give them access to credit, and they will. Read what Ben Bernanke has written on this subject. And, Many people who make enough money to get tax cuts are professionals who don't otherwise employ many people.
The Kemp Tax Cuts, in the Reagan years, were offset by government borrowing, which kept the amount of money in the economy stable. Otherwise, there would have been a downturn. Laffer suggests that an investment equal to a tax cut will result in growth. That has not happened, to date. Keynes guaranteed it by taxing at a higher rate initially, but cutting the rate at an amount equal to the level of investment. And, we had the economic growth of the 1950s which was spread across the economy, to prove it. You are right, the math is simple.
What is remarkable about Laffer is that people still believe it, simply because they want to, not becayuse the results are there.