> Why on earth do people trust a closed source messenger owned by Facebook, which backs up to Google?

Those are some of the few companies that are large enough to oppose governments.

> Signal and Matrix are open source

The main advantage of which is to enable audits like this, which WhatsApp is doing.

Of course you can't actually build WhatsApp from the audited source or pin it to the audited version... but you can't do that with Signal either. Not to mention that you're stuck with closed-source Google Play Services (or closed-source iOS) anyway.