I am not sure if I recall correctly but wasn’t there a reason where gates wouldn’t donate all of his wealth at once and needed to do it in stages ? Also, maybe they can’t utilize all that cash at once. Therefore it would be best to be illiquid until you need the liquidity.
Nobody wants that much cash in one lump sum, they want an anual income. If you give me my anual income (including benifits and supplies) and I can quit my current job to do charity work. If instead you were to give me a billion dollars, I can still quit my job to do charity work, but this time I'll be doing it from a very nice yacht or something else.
Money to charity is the same way: you are much better off funding a smaller amount over the years (as opposed to some other job they could do instead) than giving them money, either they waste the money - even prudant investing of money is a waste since it takes away time that could be used on the thing you want the charity to do.