We can go on and on about how they were lucky and it would all be true. The fact that they were born where and when they were, the connections that fell in to place, etc. are all opportunities they were afforded. But you still can't have the success that Warren Buffet and Bill Gates had without also being incredibly talented and hard working. You could then go on to say that they were born with higher than normal ability (another form of luck). However, it all just sounds so cyclical. So I'll leave you with one of my favorite quotes which I think boils it down very simply:
“Luck Is What Happens When Preparation Meets Opportunity" -Lucius Seneca (Roman Stoic Philosopher)
What is being argued is that they where lucky to be born in a society where their skills could make them millions.
I.e. they didn't create all those money in isolation, they made them in a system that made it possible.
Thats the point. Not that they aren't brilliant at what they do. Just that no one in that end of the wealth scale is that brilliant that they created purely based on their own skills.
Even more to the point, you can be incredibly hard working and intelligent, but without a good deal of luck (outside of being born into our society) you may not make it. Luck is a HUGE factor on many different levels when talking about wealth/success.
Again, not taking anything away from Buffett, just pointing out that luck is a really big deal that doesn't get much air time.
“Luck Is What Happens When Preparation Meets Opportunity" -Lucius Seneca (Roman Stoic Philosopher)