Why? Maybe Google hopes that if reCAPTCHA is sufficiently annoying and prevalent that users will disabled any and all tracking extensions they've enabled?
Probably neither explicitly as a human judgment, but both constructively - some A/B experiments noticed favorable numbers went up by doing the crappy thing, and so it has been decided.
I'm waiting for the day when it pops up "find the humans" and there's someone clearly wearing CV-camouflage. For anyone that doesn't get it: you let that human hide, because it could be you in twenty years.
