With the scientific method and anecdotal evidence: kind of never. It’s illegitimate to draw that conclusion, this way.
You got to dig down to the facts. Or we can just tell the fact that a lot of people feel that way. And I mean “confirmation bias” is a very good explanation. We also have thousands of people believe in esoterics, homeopathy etc. The mechanics of psychology are well-understood. And it’s kind of the reason why we invented science in the first place. Because we found things aren’t always as they seem. And there are a lot of dynamics to factor in.
If we want to get to the truth, we have to do a proper study. I’m not an expert on this, so I don’t know if we got to that, yet. I know people have demonstrated this is technically possible. But as far as I’m aware people have also taken apart a few of the major apps like Facebook etc, logged the traffic and couldn’t find anything that uses microphone data to do targeted advertising.
Conclusion: It’s either not there, or we missed it.
Btw, I think it’s pretty much accepted fact that smartphones do spy on everyone. It’s the main business model of any big tech company. Google, Meta… They definitely have algorithms to tailor their targeted ads to someones personal profile. And per default they look at what you’re doing online all day. Keep track of your location if they can… The one thing that’s unclear is whether they use the microphone and also listen to your offline conversations. My main point being: Listening in with the microphone isn’t that far off. If you feel uncomfortable with that, you might want to re-consider a few other things as well.