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.
It uses other signals too, like what other sites you’ve visited with that checkbox on it, what CloudFlare has seen your IP address doing in the past, etc.
The google one is able to see if you’re logged into a google account and take that into account.
There’s even a new variant of the Google captcha that is invisible and doesn’t even bother to show a checkbox.