Even if they are physically identical, that does not make them “genuine”. A genuine part will also typically come with a warranty that the seller will honor in case of a defect.

And how can you be sure that they are “real apple parts from the real Apple factory”? Because some factory on the other side of the world, who in the best case have proven that they are willing to engage in shady business practices by violating their agreement with Apple, says so? How do you know they aren’t parts that failed some quality control check? Or that they aren’t just outright counterfeit?

The thing that makes an Apple part genuine is Apple’s word, their reputation, and willingness to stand behind it.