Because it ain't private at all.

Up until recently the CIA was sponsoring the Signal Foundation by tens of milion USD each year: "Signal was originally founded with money from the US government-funded Open Technology Fund," -> https://www.wired.com/story/signal-operating-costs/

I won't even invite you to learn how their algorithm was first developed, it is even worse. Best thing you can do is really to keep your distance from fedapps.