People put misspellings into their dictionaries. They don't know how to remove the misspelling. Thus, there's no red[1] squiggly line telling them there's an error.
[1] Who chose red / green? I'm not colour blind, but I think these are not a good choice. Especially since the squiggles are the same - at least make the shapes different.
I want to say Microsoft, in Word, and green did not originally exist until later versions (if my memory serves). Not sure that they put spell check in a word processor for the first time (Wordstar?), but I think red underlining was originally a Word thing. I could be wrong.
Anybody know for sure? Now I'm curious, and it's tough to Google.
[1] Who chose red / green? I'm not colour blind, but I think these are not a good choice. Especially since the squiggles are the same - at least make the shapes different.