if the founders create the value, why would you give them a chance to leave with discounted future profits before the value is realized in a real (not discounted) profit stream?
Second, there's no valid comparison between the A-round valuation of a startup and Amazon, a giant publicly traded company with a decades-long track record and huge cash flows. Mutual and pension fund managers invest in Amazon. To a first approximation, none of those managers directly value startups.