I assumed you meant "aren't" always evil?
If alignment is subjective, yes. My PC is LG in his own mind, CG to his allies, NE to his enemies. But D&D has always played alignment is objective; my PC IS LG. He is on team law and good in the cosmic football game. And it's done that as a way of excusing the violence that is the core gameplay loop.
If mindflayers raise slaves and conquer new lands to increase its breeding stock needed to maintain its dietary needs, it's no different than a rancher that moves farther west to gain new grazing lands for its cattle. But make a module you're supposed to fight a bunch of ranchers to free their livestock and people are going to get uppity (Well, maybe not PeTA). So, you make them evil squid men with a desire for brains tartar and the game smiles at the cognitive dissonance. But to the mind-flayer, he's not much different than your rancher who also would like to eat and keeps lesser beings as its food supply. It doesn't care any more about the village it destroyed and the people it captured than the rancher did about the cattle he acquired. The difference is the mind-flayer's cattle are sapient and they look like us.
D&D's morality is already rather sus, and perhaps its only attempt at hail Mary is "but they're cosmically EVIL" to justify why you put goblins to the sword rather than engage in geopolitical diplomacy and international law. They game ain't as much fun to bust in, serve warrants, and demand the goblins show up at court at 10am. Graying up the morality further means the violence is justified even less.