Maybe they should be. Or maybe a future MM should be divided into "monsters," "humanoids," and "beasts" sections.
Say that monsters are unnatural intelligent beings that--due to being creations of gods, the result of ancient curses, being truly alien beings, or similar effects--are Mostly [Alignment] and mostly perform a specific role. There's monsters that are of different alignment and who choose different roles, but those are fairly rare. Some of those monsters may be of humanoid form, but they are not biologically or psychologically humanoid. That's more like a camouflage than anything else.
Humanoids are people. They likely will have biological tendencies towards a particular mindset but not towards good or evil. Orcs may be temperamental or prefer showing their emotions physically or by shouting them loudly, but are no more going to be murderous than anyone else is.
Beasts are animal-intelligent beings that are either natural (real world) or fantastic (griffons, etc.). Thus, some of them may be vicious predators or highly and violently territorial, but they're not evil.
Edit: There should probably be a different term than "humanoid" here, since not all people are necessarily humanoid.