Because they have, in fact, given away much of their income. Thousands of people have taken the Giving What We Can Pledge. I suppose they could all be lying bu

Because they have, in fact, given away much of their income. Thousands of people have taken the Giving What We Can Pledge. I suppose they could all be lying but it seems unlikely. (Happy to send you my donation receipts if you like.) Also, the Founder's Pledge (in which entrepreneurs commit a portion of their future proceeds from an exit) is legally-binding.