Microsoft was founded in 1975. The Gates Foundation was founded in 2000. Why did we have to wait 25 years to see any money go into good instead of Gates' pockets?
Also, Gates is still very, very rich, even after giving away a lot of his money. He also helped a lot of his friends get very, very rich, and even the ones of them who give to charity will remain very, very rich. That's billions and billions of dollars that came from crushing smaller businesses, hardening monopolies, and pulling money from governments that could have gone to aid programs.
I am not sure where you got the "saving 10 million people's lives" statistic from, but if it's true, then how many millions more would have been saved by the billions of dollars that Gates and his friends have siphoned off by being middle men? Do you really think even half the money that has gone into MS will come out in the form of aid?
If $x spent to save 10 million lives saved between 2000 and 2015 are good, wouldn't $2x spent between 1975 and 2000 have been better?