The Queen is in a league of her own - she owns a sixth of the surface of the Earth, but as has been said, she's hardly able to liquidate a lot of it any time soon. I still don't think she's particularly hard up with available funds, though. If anything, her influence is more powerful than her vast wealth. Considering that her people 'make it known' to Daimler that HM 'would be interested in considering' one of their vehicles and then a top-of-the-range one is delivered the next morning, on the house, I do wonder if the things that 'she'd be interested in considering' couldn't be upscaled somewhat - water, sanitation, basic shelter for those who have none etc.
As for other supremely rich people who don't have the trappings of state or position and who simply have billions of pounds or dollars at their disposal, I simply cannot get my head around why most of them wouldn't want to help. £100m is surely more than anybody could possibly spend in the most opulent lifetime, so why would you want to keep £10bn, when you could do massive good worldwide for £5bn and still be left with far more than you could ever possibly spend? After a certain stage, it ceases to be money - i.e. a tool with which to buy things - and just becomes an ever-increasing number.
We're not on a water meter, so we could take thousands of gallons a day for no extra cost - but we wouldn't do that, because we simply don't need it and cannot use it anyway. Why would we leave a hose on at full power 24/7 poked down a drain "because it's ours by right and we've paid for it" when we can just take as much as we want to use, never remotely coming anywhere near going short, and then leave the rest for others to use, who do need/want it. It's the same principle really.