Zach Holman recently blogged about this:

I saw GitHub’s valuation move from sub-$10M to more than $2B; beyond that, GitHub is turning a decade old this year. Traditionally, this would be a great point where lots of little companies get spun off from this created wealth, and employees and former employees would be able to help with that.

In reality, I can probably count on one hand the number of people from GitHub who are in a financial position to become true angels. This is far from a problem unique to GitHub; the entire industry is concentrating its cash in a select few.

https://zachholman.com/posts/slow-exits

HN discussion: https://news.ycombinator.com/item?id=13655855