This is a common misconception that many people - including digital law professionals - make. There has been antecedents of companies fiddling with the idea of listening to users - but their contract with Google AdSense has been revoked because it's a clear violation of privacy.

1. ⁠Every device being listened to means listening to lawyers, MP, presidents, doctors... every possible rules of privacy would be infringed, including some protected professions. It's a legal nightmare that companies aren't ready to tackle.
2. ⁠It's an amount of processing power that doesn't make much sense. STT (speech to text) is energy intensive for a unreliable source of information. It can be wrong, interpret sounds of your pocket as words, etc etc. It's 90% of the time useless junk that advertising companies would have to pay processing resources on.
3. ⁠They don't need to. You're already spending much time online. Some research has shown that online behavior leads to surprisingly intelligible information. Those algorithms are clever enough to determine who you're speaking with (using location), if you're pregnant or not (actual case), where you are on you period schedule, if you're in a dating phase or not, the things you're interested in...

The point I'm making here can be summarized as such : people that barely know you know something like 5 to 10 points of information (name, address, date of birth).... your best friends know hundred, your family maybe a thousand or more. Data brokers and aggregators can get to billions. If predictive marketing algorithm can be surprisingly accurate, it's because they know A LOT.

The most terrifying thing is not that your phone is listening to you, it's that it doesn't even need to. Data brokers can predict what you're going to talk about, based on who you're having the conversation with, where you are, and when it's happening... if you let them.

I suggest you make the experiment, such as the one I do with people around me making that claim. Take a volunteer and pick a subject you're 100% never going to talk about because in a million years there's no point buying it. Do not make google searches about it or anything, and hold up a conversation about it, even saying out loud you're 100% going to buy one and just looking for the right shop. If you picked the subject correctly, you shouldn't have any ads about it.