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.
Why? Like, just why go through all these hoops?
What is Evil even meaning here? If people want to say 'humanity, on the whole of history, is more evil than not' and I think an argument can certainly be made, then....what's the big deal if Dragons are most of the time 50.1%, evil?
What does it matter if gnolls, are evil, most (50.1%) of the time are evil?
What does evil mean?
If most of the time (again, 50.1%) Orcs encountered are reavers, and evil, within a setting, so what?
Don't like it? Well there's Eberron to show a different way, go nuts.
Why, does this matter at all when you can do whatever you want at your table, and there is a setting (if not more) which actively promotes the design principles, morality, and world view you seem to believe we ALL must accept as the one true truth.
My Orcs rule a civilized nation. It's a brutal dictatorship. They care for their people, and slaughter anyone not them.
Still not good enough because it's a monolithic culture I'm sure.
So what is? Amorphous, 'anything goes'. Eberron style cosmopolitan settings.
Hard pass.