Not convinced working for government is the best use of your potential, though considering he owns 5% of all US Treasury Bills, I wouldn't say his impact is any less than being a government employee.
That money has already been committed to charitable use for the most part, or already donated. He is essentially managing billions of dollars on behalf of charitable causes. Seems like a pretty good use of his money management skills.
Besides all that he has advocated for that money to be taxed away from him for decades.
He pretty famously lives below his means in a normal suburban house and drives around in a car that most software engineers could afford (Cadillac sedan).
From a societal perspective, I’m not sure you could ask for much more from any one man.
Well I don't think him sitting on billions of dollars at 94 is better from a societal perspective.