Mostly because that's not really a grammatical error: http://ngrams.googlelabs.com/graph?content=girl+that,girl+who,man+that,man+who&year_start=1800&year_end=2

Mostly because that's not really a grammatical error: http://ngrams.googlelabs.com/graph?content=girl+that,girl+who,man+that,man+who&year_start=1800&year_end=2000&corpus=0&smoothing=3 http://ngrams.googlelabs.com/graph?content=girl+that,girl+wh... Middle school English teachers love to invent simple rules of the language that don't reflect actual usage very well. The choice between "that" and "who" as relativisers is subtle, and the animacy constraint doesn't explain the facts of how people speak.