Great points, I think I agree on all of them. Cynically speaking, I think the answer is just that it's a round psychological number -- the three comma club. It'

Great points, I think I agree on all of them. Cynically speaking, I think the answer is just that it's a round psychological number -- the three comma club. It's a rough binary signal that indicates growth and clout to a layman, and works reasonably well as a proxy therein, even if it's pretty noisy. I've personally noticed that working at unicorn companies has done a lot more for my career and network than non-unicorn companies, and it's done the same for lots of folks I worked with there, even those who weren't on as high profile projects as me and who felt like they didn't grow as much.