Nobody has caught continuous audio feeds being transmitted from smart devices to the cloud (which would be noticeable due to increased network traffic and bandwidth usage) nor identified any secret speech recognition code on the client (which would be noticeable due to severely shortened battery life). Nobody who's worked in adtech has come forward to blow the whistle or admit that they shipped this feature for a big tech company.
I get why it's an appealing conspiracy from a gut instinct perspective, but it really makes no sense. When you're observing the behavior of billions of people and using machine learning algorithms trained to get the best results possible, some uncanny shit will naturally result. Look at how effective LLMs like ChatGPT have gotten without an obvious route to profitability, then think about how much more money has been invested into ad targeting algorithms just in the last couple decades alone.