Because Amazons stock price is valued strongly based on future potential. For Bezos to cash out now and do philanthropy, without a very strong replacement to realize Amazons potential, shares would fall 50% effectively halving Bezos' worth.

Regarding your second question, it is actually a similar scenario to when Gates left Microsoft. Right before Gates left, Microsoft was at one of the highest peak values, $58, almost it's Market cap today if you account for inflation, and a year after Gates left the stock had plummeted to $24 a share. Many would propose that Bezos leaving would actually be more detrimental, as Microsoft at the time was not a growth stock, but a well established money printing machine, while Amazon stock is still banking on future potential.