Because investors have no clue how to value growth companies, because interest rates are low and because some prominent growth companies with no profits have been on a non-stop* share price rampage for 14 years. The revenue multiple pricing of Amazon, Facebook, Twitter et.al. is cargo cult finance. No one really knows how much these companies will eventually earn, so some commentators have come up with a plausible after-the-fact metric that sounds good in a soundbite.

A company is worth the present-value adjusted sum of all its future earnings, and that is it. How to guess, discount and risk-adjust this number is a different question.