Imagine this scenario to help understand why:
Let's say the top richest people gave away so much money that every person on earth was a millionare.
What would happen?
People wouldn't change their spending and other habits, in fact they would buy more useless stuff.
Prices would thus rise and rise until they matched the new million-fold buying power. (Think about this: Would you keep your pretzels at $0.50 if you knew everyone was a millionare? Ofc not.)
And then? Then we would be right back to where we started: Most people would be just as poor as before, and the rich would be just as rich as before.
To the downvoters: I'm genuinely interested in your serious rebuttals of my hypothesized scenario.