Ask YC: Why don't you take a convertible note instead of equity?

It seems strange to ask, but given all the changes pushed by PG within seed stage investments, why doesn't YC take a convertible note? Why push one set of standards for other early investors but extract the highest price from young companies? I get the fact that 7% doesn't require all that much in increased valuation to pay for itself. But it seems strange to require the equivalent of a ~$250k valuation to participate and with a take it or leave it offer. Startups may rarely be bound to this valuation in subsequent financing but a $1M valuation would still give YC 1-2% equity on average without any discount. It seems strange that PG pushes for notes but then relies on equity. Why?