Opportunity cost - assuming everyone has fair market cash wages, the BigCos are offering more in bonuses and stock - at GOOG/FB it's not uncommon to see $150-200k+ per year in stock, with less uncertainty.
You'd expect that for taking on the risk (relatively low as it may be for a unicorn) you'd do better than the extremely low-risk public stock packages being offered by BigCos.
Correct, clearly not every engineer at GOOG is getting it but it's also not rare. And not particularly high-level engineers either - I'm talking almost exclusively about level 4/5 SWEs.
I also know some people at other BigCos getting similar levels (to be clear: $300-400k/yr total comp of cash + public stock) so it's not just a GOOG/FB thing.
You'd expect that for taking on the risk (relatively low as it may be for a unicorn) you'd do better than the extremely low-risk public stock packages being offered by BigCos.