You miss the part where Bill Gates created that wealth. He didn't take wealth that other people had already. Also, Microsoft employees collectively earned many times what Bill Gates did. And Microsoft customers used the software and hardware to generate many, many times more wealth, which went to their employees and customers, etc. Each sale (well, most sales) of the software benefited both Bill Gates and the person/company buying the software. Multiply the benefit by billions of sales, and you've got billions of dollars.

how many have been unable to afford higher education, how many people were denied the opportunity to contribute to the world because they were not born in a middle upper class family as Gates was.

A lot fewer, thanks to Gates & Microsoft.