Because it isn't in the list of accepted words[0] (most used words) as inspired by XKCD[1].

[0] https://splasho.com/upgoer5/phpspellcheck/dictionaries/1000....

[1] https://xkcd.com/1133/

Edit: I just figured out you might fully realize that and were probably just pointing it out to OP. Ignore me. :)