Ask PG/YC: Why doesn't YC take a convertible note instead of equity?

With the emphasis on convertible notes for startups (http://paulgraham.com/hiresfund.html), why doesn't YC take one? Why push one set of standards for other early investors but extract another for yourselves?

$20k may not be much, but in the context of a convertible note, even with a significant discount, YC's contributions could be properly valued by the market rather than set as an arbitrary standard.