TL;DW: CAPTCHA isn’t designed to identify if you’re a human, it’s designed to identify which specific human you are. And if it can’t do that (because you’re not already part of the AdSense ecosystem making Google money), they force you to do free labor by helping Google train their image identification algorithms.
It’s also complete and utter bullshit: https://m.youtube.com/watch?v=VTsBP21-XpI
TL;DW: CAPTCHA isn’t designed to identify if you’re a human, it’s designed to identify which specific human you are. And if it can’t do that (because you’re not already part of the AdSense ecosystem making Google money), they force you to do free labor by helping Google train their image identification algorithms.