Why Do Small Donors Give Now, But Large Donors Give Later?

I like the idea, but I'm not sure it fully captures what is going on. We could be comparing the poor person not to the foundation but to the rich person who endowed it, and asking why they waited until late in life to do so rather than continually donating. The 'poor' person does indeed have a valuable asset in their future earning potential, but so does the young Bill Gates. He could have sold a bit more MSFT stock every time it went up, rather than waiting until the end.

It's a clever explanation, but I'm not sure how much to believe it without analysing other hypotheses. E.g. maybe tax-deductibility is a major factor, or maybe it's just much harder to give away large amounts of money quickly.