I think more likely is that bill and Jeff built their wealth through their businesses, which they are still directly involved in. Giving away their wealth would be equivalent to giving away the business they built from the ground up

I think there's also a question of efficacy. Bill and Jeff are really good at turning $1 into $10. Melinda and Mackenzie? Not so much. If bill or Jeff wanted to maximize the amount they can give, the best strategy would be for them to continually earn until they die, and then donate it.