They're not secretive about it
The article explains that this is part of what reCAPTCHA does, e.g.:
> Finally they combine all of this data with their knowledge of the person using the computer. Almost everyone on the Internet uses something owned by Google – search, mail, ads, maps – and as you know Google Tracks All Of Your Things. When you click that checkbox, Google reviews your browser history to see if it looks convincingly human.
But your point is otherwise right in that it's used for ML training, which Google admits as another commenter pointed out.
Human [n]: Entity that uses Google®-brand services.
— Google Dictionary, 2020 edition
Or, maybe they feel I'm not pulling my own weight, seeing as I rarely ever click on adds. They probably need more monkeys to feed the beast so I get selected to train their AI beast.
Edit: It could also be that I'm always running on incognito mode.
With the possibility of almost uniquely identifying us on the web through fingerprinting... Google, of all companies is in the perfect position to know that my web request was made by me... And therefore I'm not a robot.
You can only conclude that recaptcha is a ml training exercise.