Even very large salaries are probably not a very significant amount of wealth for these people. I think this is a case where "all large numbers look the same."

If Warren just directly gave his money to his three children equally, and paid 40% inheritance tax on that money, and then the children each just used 1% of that wealth per year, then they'd each get:

144000000000 * 0.6 / 3 / 100 = $288 million each year.

No foundation pays its CEO 10% of that much money. I think we may have to accept that in fact this is a way to give money to charitable causes, not just horde it.

I wonder if they could even unwind the fortune to pay 40% in cash.

In this case yes. However that is not the case for all charities and so you need to look.