These unicorns are in a unique position to tell the shareholders -- whom are, let's face it, increasingly acting like a cartel across wide swathes of the economy because there are fewer and fewer of them each fiscal year -- to tell them to fuck off.
And that scares the regulators because they're paid to keep "everybody in the fold" by extolling the virtues of being publicly owned.
Here's a useful tidbit: the mysterious market forces -- the "shareholders" as they're known -- are actually a small group of individuals or groups that pull the levers of the economy. The regulators rely on these lever-pullers to play ball when need be.
Frankly I think these unicorns are right to fear buying into the US economy by going public, the overt assertions of power by the US government are definitely something to avoid.