Evil, in Dungeons & Dragons (and most fantasy rpgs) is an objective, independent force. It exists. Period. There are creatures that are, literal, embodiments of Evil. There are deities who do the same. There is Magic that, again, objectively, IS Evil. It merely IS. Like Good. Like Neutrality. Everyone with their situational ethics and desires to annul Alignment as a game element, let alone mechanic, seems to have a real problem with wrapping their heads around this.
There is OBJECTIVE Evil, as a force, as an "energy" (if you prefer), a magic, and, yes, creatures (and deities which are simply bigger/immortal/more powerful "creatures"). They ARE Evil.
HUMANS "choose" to be Evil because humans CAN choose. They have free-will. Other species who one fluffs (r lore states) are "free-willed" can CHOOSE to be Evil or not. NOT any/all creatures have this freedom of will, the option of "choice."
There is zero reason to assert that, because humans can, that means all fictional fantastical creatures can or "should be allowed to." Goblinoids? Evil. Drow? Evil. Orcs? Evil. Hill, Frost and Fire Giants? Evil. The Evil is "innate," it's "baked in." For the magically challenged, it's in some added (or removed) DNA. They do NOT have the choice/option.
NOR, importantly, do they necessarily WANT the option. A devil does not WANT to become an angel. An Ettin does not WANT to "be kind" to the halfling about to be its lunch (nor will it feel any remorse about treating it as and having it for "lunch"). They do not philosophize or opine on their "nature" or the Nature of Evil (or Good). Evil IS what they are and know. It is Power. It is what exists in the complete absence of Good. It is to be obeyed, and feared, and exerted over others. Whether you tell yourself it is for "their own good" [LE], pure selfish gain/ends [NE], or because of some inherent cruelty and destructive nature that "just likes to watch things squirm and suffer" [CE], Evil exists in and of itself. It is simply not in the "spirits" (or "souls" if you give such creatures "souls") of certain beings.
It is entirely possible, and ridiculously simple, to say "Orcs are one of those innately evil species." Giving EVERYthing in a world a conscience and free-will is counter to the adversarial and heroic nature of the game...and, well, impractical to a setting's internal consistency, in addition to everything else.