A) whatsapp improves the daily lives of hundreds of millions of people. You don't necessarily need an absurd customer lifetime value to get a $19b valuation with that many people using your product.
B) SpaceX provides almost no current value to anybody. And the people it does provide value to are "free riders" in the sense that they love watching space travel and get to watch SpaceX regardless of whether or not they pay for it.
Just an inexact thought experiment. Let's say we poll the roughly 1b people living in first world, and ask them each to pay $10 to see SpaceX do whatever SpaceX does. How many people even think it's worth $10?