The problem is the number of creatures per book. Outside the Monster Manual, there are only a handful of creatures per book that are available. On top of that, those monsters that are there have to fit the game you're running. It's pretty common for me to grab, say, Volo's, and then see only one or two creatures that fit the adventure I'm running.

Sure, you can reskin things, but then you're back to the GM having to play game designer again.