The simplicity of it is logic defying. It used to be that you had to find crosswalks or move puzzle pieces or type blurred letters and numbers, but NOW all the sudden I can just click a box and HEY!, I'm human?
That's hardly the Turing Test I'd expected.
The timing of the click captcha loading is randomized and it probably is looking for human-ish cursor movement? (Like you're probably moving your hand in imperceptibly small ways that are difficult to replicate). Clicking before it loads and doing it repeatedly probably triggers detection.
I used to think it was timing based, but now leaning on the idea that it just performs more fingerprinting in the background: user agent per ip pool, canvas or puppeteer checks.
This is correct. Those captchas are tracking everything they can and comparing it to other results to try and figure this out. Mouse movement, delay before you click, everything.