So... Evil is irrelevant.
Antagonistic is the point. It's also why Tieflings are so much more popular than Aasimar.
See... you don't have to be evil to be the bad guy. Klingons aren't evil. Most of them are jerks. But we get Worf on the Enterprise D, anyhow, to show that the antagonists that the Federation has been fighting throughout the Original Series and movies aren't actually Evil. Sort of the same thing with Seven of Nine and Borg, even though they're much closer to capital E evil than the Klingons we've gotten a lot of exploration of their identity and stuff through Picard to show us that they could even be a force for good.
Orcs don't need to be CE as a race or culture to be antagonists to various kingdoms. And the birth of their children can thus still have various stigmas that replicate a specific upbringing that a player might wish to embody in their roleplay. One of being the social outcast because they're considered "Bad" or "Ugly" or "Stupid" or whatever other epithet is applied to this half-caste character.
Could someone play an Orc, instead? Sure. No problem. No reason not to. Unless the societies in the game space wouldn't tolerate having a full blood orc walking through town carrying a giant axe for... some reason. Possibly relating to warfare or other ongoing struggles...
But a half orc? Well. They're easier to stomach for most people in such a setting. Easier. But not Easy.