I do wonder why though. For a long time, I assumed it was a rate limiter, but then another HN commenter pointed out to me that time is more valuable to humans than bots. Bots can work on multiple captcha's in parallel.
I've become accustomed to just closing any page that presents me with a v2 reCAPTCHA.
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.
Edit: Unless, it occurs to me now, Google is monitoring how the user responds to the fade in. When in the fade process do they click the square, for instance.
Still seems like it wouldn't be all that helpful, though.