So to be recognized by name. If you said, "Hey do you know the author Katherine Grant?" People would look at you funny. But if you say, "Hey do you know the author K.A. Applegate?" They light up and say, "Yeah, she did those animal morphing kids, right?"
It's the same reason Angelina Jolie never changed her name when she was with Billy Bob or Brad Pitt.
They're known by their name. Without that name, no recognition, so no book sales.
