And what did they achieve?

It was mostly PR. Gates and Buffet were (and are) rich, but their money simply does not come close to the scale needed to fix the world's problems.

Gates is worth $110 billion. Global GDP is $126 trillion.

There was a lot of PR, but the money Gates and Buffet gave away, the foundations they created, the attention they drummed up for various charities and causes is certainly not to be scoffed at.

I'm sure they could have done more with less fanfare, but you seem to be setting the bar at "fixing the world's problems", which seems unfair. They have undoubtedly changed more lives than I'll ever manage to.