You see this discourse come up from time to time in other fantasy settings. I remember there being quite a stir over whether lizardfolk in other media should have breasts.
I'm not deep into the Warhammer lore, but there's a few things I know about orkz:
They grow from spores like a fungus
They were manufactured by some other race as a sort of guardian caste
They have potent psychic powers that manipulate reality around them
My theory is that the folks who created the orkz had nipples themselves. Impressionable young orkz saw that their creators were fully grown and had nipples, so maybe they thought they ought to get some as they mature. If they believe that should happen, then it'll find a way to happen.
Later orkz would have seen that the previous generation of orkz had nipples, so they would continued growing in this way.
They have potent psychic powers that manipulate reality around them
I read once that this is why they paint their vehicles red. Because they believe that red makes it go faster, and with their latent psychic ability, the red ships actually DO go faster, because they believe so strongly in it.
Oh sure, but the rumors of the Bale Eye existed before the implant did. That thing is only about as strong as a laspistol. Sure it'll burn a hole through your trousers but it's isn't about to explode a charging Ork into bloody chunks. All that power comes from the gestalt field, presumably.
There's also the fan theory that the only thing keeping the Emperor barely alive is the combined power of all Orks who consider him a worthy and immortal opponent.
One of the old codex books. It's going back decades however. The works have changed a lot from that era. It was the same codex as the buzzer squig catapult, if that helps narrow it down.
... honestly, the most likely argument is that they saw other races and collectively assumed they ought to have nipples too (oi! why dem pink skins 'ave dots on em? Orkzz ought 'ave em too!! wauuugh) also and so they manifested them. now its just part of their genetic code and none of them ask why anymore.
Orkz are no strangers to krumpin' gitz because they aren't Orky enough though. Even their own clan. I find it hard to believe they'd be easily peer pressured into having nipples, let alone by other species.