It's such an interesting phenomenon that so many ultra rich people are essentially just hoarding wealth beyond what they should reasonably be able to even have use of in multiple generations. Worse, some of them simply cannot seem to get enough and will literally commit crimes and/or do indisputably morally wrong things to get even more.
I would personally never ask anyone this, and I wouldn't expect anyone who could answer it to actually answer it, but I think what komali2 asked is one of the most interesting questions out there.
You did mention something I didn't think of which is lifetimes, I guess if someone wanted to guarantee an ultra wealthy lifestyle for all generations of their kids and grandkids forever, that would be a reason to hoard wealth into the hundreds of millions.