The whole Pixiv Fantasia thing started out as an art meme by ArohaJ. Basically, you draw your character and then your submission would be randomly entered into one of the countries. There’s a general storyline and artists can input their own events as well. The goal is to get more points for your group by contributing additional illustrations (for example, drawing your character and others’ in the middle of an event like a battle scene). So yeah, most, if not all, of the illustrations were probably not drawn in mind for being printed in an artbook in the first place.