I'm not sure he's actually the philanthropist with the "highest positive impact", when looking at the "net value"

he's "extorted" a lot of money from various states by locking and price-gouging, money that would have otherwise been spent on social projects

basically he has done

Gates -> extort money -> fantastic personal wealth -> gave back to organization *he* decides to give too

while the normal path would be

Governments and people have lower spending because they don't need to give Microsoft too much cash -> governments and people decide by themselves how to spend extra money -> there are more, and more diversified, humanitarian actions