I'm not convinced this isn't explained entirely by compensation. There are perhaps a dozen large tech companies at which an only moderately remarkable engineer can make $500k a year right now. That number has gotten bigger every year for the last decade at least, and the much larger pool of "every other software company" is increasingly falling behind. You should follow up with your exits a few years down the line and see where they end up long term. I bet a few of them will have retired - how often do your engineers retire at 35?