Another factor is that contemporary companies more often will issue RSUs instead of options. This (afaik) allows a company to avoid hitting the shareholder threshold which would require reporting.

IIRC, Facebook didn't have to go public when they did, but once they hit the threshold they would have to start reporting, and at that point there's not much reason to not be a public company.

(Another reason cited was allowing early employees to take advantage of the value of their shares without otherwise having to leave the company.)