A large fraction doesn't not mean "most". 1/4 is a large fraction. 1/10 is still a large fraction. Due to the large amount of people companies like fb, goog, apple etc employ, statistically you should expect that a SV startup on the verge of not being a startup anymore employs some ex-ers. Not having any, will raise questions. This doesn't mean the startup is bad, the answer to said questions might...
Not just "work for" but "have ever worked for" which is, at this point, a much larger number.
What percentage of developers that is I haven't the faintest clue. But definitely significantly more than 10% of my friends have worked for at least one of FAAMNG. And I have a lot of non-nerd friends too.
The propinquity is insane. When my GF decided to leave MS she walked from her MS building on her lunch break to her interviews at Google and LinkedIn (LI has subsequently moved). Of course the inverse is true too: there aren't many startups in that area, except in the Landings complex (where Cygnus used to be!).
