Different reasons for different people I suppose. Lazy people wont bother to spend energy correcting. One who is inept at language could miss mistakes. Inability to form thoughts into words, and/or only caring about getting ones message across thus not bothering with corrections.
True, but the article points out that people "hear" the words they're typing and will sometimes type a word which sounds the same instead of the one they meant to, especially if it's a word they type more often than the other.