Although the Island of Myst was supposed to be Atrus and his family's home, there were no actual living quarters (e.g. Bedrooms, Kitchens, etc.). No place for them to sleep or eat.
At the time Sirrus and Achenar became trapped in the red and blue books, both Atrus and Catherine were unable to return to Myst. So who was left to tear the pages from the red and blue books, and scatter them across the Myst worlds *after* Sirrus and Achenar were inside?