I generally attribute this kind of thing (which I see often too—including many instances of this particular aberration right here on fluther) to ignorance and poor attitude, which in my mind are very closely linked. (What they don’t know doesn’t matter because, of course, they are doing fine without it.)
I have seen so many native-English-speaking Americans who are careless, sloppy, or hopeless spellers that I don’t think we need to look to English learners (many of whom are extremely meticulous with the language) or people with learning disabilities to find an explanation. You’d be amazed how many such folks are earning their living as professional writers.
It’s not a far stretch from “definately” and “definatly” to “defiantly,” after all, and I suppose “defiantly” must have the advantage of looking somewhat familiar.
Many people seem to look only at the beginning of a word and then wing it from there, leaning heavily on context to support meaning. They expect the reader to do most of the work instead of sharing the burden of communication.
I do wonder what those folks see or think they see when they read. Do they suppose the Tony Curtis-Sidney Poitier movie is about two people who are really definite about things?