Same reason 4chan reCaptcha is no longer used to digitize the other word. If they had their way the correct answer to all reCaptchas would eventually become "nigger nigger"[1].
Fun times.
[1] the idea was to always write the correct word and just enter "nigger" as the other word. This eventually led to reCaptcha disregarding 4chan answers from the pool of resolving weird words.
You aren't supposed to realize it or do it. Google uses the human's work on solving the unknown word to either get street numbers from photos or digitize books.
However, the whole point of them having the humans do that work is /they do not know the correct answer/. Since they do not know the correct answer they cannot be basing the test of the CAPTCHA on it. From there it is not a big leap to surmise that you are only actually being tested on the word that actually looks distorted on purpose.
Either you just know because internet or you get it from trial and error.
It's probably written on recaptcha website or wikipedia article, but basically you get presented one word that machine can't read (the distorted letters) and something out of a book that google had digitized and uses you to ocr it. You also get a lot of address numbers for google street view.
Just input the distorted word and type anything for the other, and it will work. Of course the instructions won't tell you that or else people would act accordingly and google would lose this free labor source for the tedious work of proofreading digitized version of books.
>I would expect people to know how reCaptcha works.
Why should you even expect that? Recaptcha was interesting in 2006, but anyone not following the news around that time or around the its acquisition by Google might not have learned this.
