I'll take a steady $300,000/year job over the chance of winning a startup lottery any day of the week.
You have to be lucky just to be in a startup that succeeds (because most of them flop). You have to be in from the beginning, otherwise you get very little equity. And even if you win, you have to pay taxes through the nose (in California, 26% AMT + 9.3% state tax).
Some concrete numbers.
In 2004-07, there was an average of 160 IPO's per year in the United States. There were almost no IPOs in 2001-03, and again since 2008. Let's assume that, for every IPO, there was one acquisition of a private startup. I'm going to guess that each IPO created an average of ten engineer-millionaires and ten management-type-millionaires.
That's 12 thousand engineers turned millionaires in ten years, across all engineering specialties (software, hardware, telecom, ...)
BLS says that there are 857,000 computer software engineers and 291,000 electrical/electronics engineers in the country.