Because the founders have created the business and have created several million dollars of value (based on the VC's own valuation). Why should they not be able to cash out for a lifetime of financial security?
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.