Whenever you have huge fixed capital costs and high per-unit costs your upside is limited. A 10x increase in the whatsapp user base doesn't increase costs much, while SpaceX probably couldn't deliver on a 2x increase in volume for a few years. That's just the handicap of a business that makes real things.
What investors are looking at is potential. The upside for heavy-industry SpaceX is far more constrained than that of a social media company.
A 10x increase in the user base of WhatsApp would make their user base every single human on Earth, give or take a couple hundred million. Probably not going to happen. And even with 1/10 of Earth as its user base, it was making <$1B/year.
I'd argue that the upside of making launches happen at 1/10 the traditional cost, with the potential for easy reusability with minimal refurbishment (which will allow much more than 2x launch volume, and a much greater cost reduction) is larger than WhatsApp's to a hilarious degree.
What investors are looking at is potential. The upside for heavy-industry SpaceX is far more constrained than that of a social media company.