Same reason people can’t get their/they’re/there right, and people actually type out “could of”. The written version doesn’t match with the way certain accents pronounce the words, and people try to write what they say.

English is not exactly consistent in the way it spells words (the “k” in “knight” hasn’t been pronounced for hundreds of years, “though” and “cough” don’t rhyme despite their similar endings, are pronounced like “doh” and “coff”, all because of things like the great vowel shift and the lack of a spelling reform after).

That said, “mastodon” was a word in the English dictionary long before it was a social media platform. In order to type mastadon, you need to override autocorrect and ignore the red squiggles that tell you your spelling is wrong. I don’t really get that.