The thing is, building a billion dollar business doesn't require 1000x the work or entail 1000x the stress of building a million dollar one
The key thing is, a billion dollars is nothing like 1000x better than a million. In fact, I suspect it's barely 1.1x better (and there's even some weak evidence from subjective wellbeing studies that it could be worse).
The St Petersburg Paradox[1] is a fun application of this fact — the fact that when you already have a LOT of money, even a LOT more money is pretty well useless to you.