Even if engineers hit the startup lottery, the payout for most are not that much. Very few end with multi million dollar payouts.
Statistically, financials are better with big company offers for most engineers.
Some engineers that get in early with a unicorn can hit the jackpot. This is mostly combination of timing and luck and network. Some just happened to be in the right place at the right time with right group of people. PayPal engineers, early Google engineers.
Early Apple engineers were not so lucky.