Every cellphone in the world has a microphone that could be listening all the time and sending data anywhere. So does most every computer. It's a better threat vector by 1000x, more stealthy, easier to conceal traffic. But all anyone ever talks about is a device designed to listen to you talk because hey, so obvious, big brother MUST be listening in there!
Here’s a parallel construct: It would be trivial for Microsoft to have a key logger in Windows that sent every keystroke to Redmond.
Why does anyone trust that they are not doing it?
Indeed, they could scan your computer for whatever data they like and send it to Redmond. But no one even suspects it, or at least at nowhere near the level people seem to distrust Amazon and Google over sending your voice to them full-time.
This is what I mean by Misplaced Distrust. We already trust Google and Amazon with far more of our personal data without a thought.
But a microphone? Ooh, scary!
Non-techie users would absolutely notice that their data and battery is being used up in the background pretty quickly. Further, the cell networks simply could not support that kind of usage from every single subscriber at the same time.
The feds have been using cellphones as full-bore wiretaps since the early 2000s when they used it on mobster's "dumbphones". I'm sure they've figured out clever exfiltration techniques on smartphones by now.
Especially considering how willing the ISPs/telecom companies are to bend over backwards to hide surveillance. Even 3g/4g wiretapping is probably feasible.