>Wut? Have you actually ever met a person who can't?

I have met a few, yes, including people who are intellectually limited (colloquially called "dumb"), people who were born or acquired mental handicaps, people who are suffering most likely suffering from dyscalculia (dyslexia for arithmetic/numbers). E.g. we think my grandma might have had a milder form of the latter, as she had a lot of trouble adding up numbers in particular, which would have made this CAPTCHA very hard for her - but not impossible. And a friend of my mom has Turner syndrome, which is linked to dyscalculia, and she is indeed very bad at basic math.