Because if Facebook was actually listening to your mic we would know it. They wouldn't be able to hide and it would be a massive scandal. It's simply too conspiratorial to take seriously when there is no evidence other than anecdotal stories like this.
You don't see any difference between Alexa (which wouldn't work without listening to everything) and Facebook secretly listening without your consent to use it for ad targeting? None of your links provide any evidence this would be the case, or even mention Facebook.
