And this is why: so that you don’t scatter all of your monsters throughout the book. And before you ask, When would this matter? The answer is, it matters when you know you need a dragon, or a giant, or whatever for an encounter – but you don’t know which kind of monster would best suit it. It’s much easier to figure out when your dragons are all organized as Dragon (Green), Dragon (Red), Dragon (White) or whatever.
You get the impression that WoTC kind of resents still having to sell actual books.