An example of the issue.
I had the idea a few weeks ago of making a bunch of NPCs. All the major classes at Challenge 1, 4, 8, 12, 16, and 20. Because a lot of classes just aren't in the MM. And a cheatsheet of the major PC and monster races. Like page 282 but with necessary mechanics written out instead of just listed. So you could make a challenge 8 elf rogue or challenge 16 dragonborn monk in seconds.
Super handy, right?
But it'd copy a tonne of monster and PC text, even if I edited things down and focused on key combat options. And I'm not sure WotC would be cool with that.