WhatsApp adds almost zero because there are many other options to text everybody. So it's just another company that got traction. It could die tomorrow and nobody would worry much. It can be easily replaced. There is no long lasting value (again, due to so many other options and the easiness of creating it from scratch again). Text chatting is a exhausted marketed, in terms of innovation. If WhatsApp is secretly working on hologram technology then it'd be another entirely different matter, but I doubt that's their focus.

SpaceX, on the other hand, is making breakthroughs in engineering and science. What they create will be with us for a long time, can yield multiple patents, can feed into other inventions.. I think it's very difficult to argue that creating technology that allows us to go to other planets has little value. I think it has a lot.

So I think we're substituting the question "where should humanity invest money?" for "which companies are correctly valued". The latter is much easier to answer based on economics. The former, quite difficult to agree on.