Because that is a myth. If that existed, you could build an entire startup with a single developer.
I think the real 50x engineer is a person who is a strong engineer, also does design, innovates, and can build a business like a CEO. That person could actually bring 50x value to a small org.
I myself am the sole developer for a B2B software company with $5M ARR.
I'm not 50x (or even 10x) and don't think those designations are based in reality, but your second sentence doesn't follow from the first.