Among people, who aren't straight up unable due to handicaps, who couldn't solve them? It seems easy enough that mostly anyone could.
They are quite common problems with a wide spectrum of severity.
Back when you could take your car to a local garage, your car would very likely be repaired by a very clever person who was dyslexic (even dyscalculate) because as a career, men in particular who struggled with written language (or even modestly complex maths -- differential gear yes, differential equation no) could do it and thrive at it.
Should that person be treated as handicapped if they need to fill in a form? Or should forms not be designed to make people waste their time with puzzles or kafkaesque processes of any kind?
Summing dice together is not the only ability you need to solve this puzzle.
It's not the totality of what is being tested.
In other words, it's instructive to consider why a computer struggles to solve it with this particular photographic presentation, and why a person might.
“If you cannot lift 100 lbs over your head, should you be operating a fork lift?”
There was (probably) a point in history where AirBnB existed to make things easier. Now I'm pretty sure it's just there to make money.
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.