They can already do that with most of the information they already have, they don't need to listen for that type of thing (though they still might be as you see here, data is data after all).

Most people are not that unique. They know where you are and who you are with and any searching or sites you have been to and what is in your text messages or whatsapp as well as that same information for all the people 6 degrees away from you. They know people like you, who are your age and like the things you like and are where you are with friends like yours etc etc it goes on and on.

People rarely talk about things unprompted and you will only notice when you think you talked about something and then see an ad for it so there is a confirmation bias as well.