When it comes to startups in particular, it might be a case of actual shortage. Not everyone wants to work at a startup, particularly in the earlier stages, even if comp is similar to what one could get in a public company. Couple that with the relative rarity of actually good engineers among the population of qualified software engineers, the fact that you don't want to hire juniors or new grads at very early stages, and the general difficulty of hiring SWEs, and, although I'd like to see data before making a definitive statement, I can see how it could be many times more difficult to hire at a small startup than a larger and more established company.

Does anybody happen to know whether data on this actually exists or not?