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.
Proof of work, which becomes computationally expensive to scale, along with other heuristics based on your browser and page interaction. I believe it’s less about clicking the box and what happens after you’ve clicked the box.