Well, the movements aren't EXACTLY the same. Yes the movement are still the same family of movements, same brand, but they aren't certified like the ones that are placed in the watches.
You are paying for a lot of things when you pay for a gen. The name, of course, is the first. The movements inside are based on the ETA movements we all have in our watches, however they also have modifications which make them different. For instance, the Co-Axial movement in the Omegas with the increased power reserve and modified with the co-axial escapement. Rolex uses their own in house movements, so they will not be the same as the reps. Other companies, such as Panerai, also used ETA movements, with modifications. The Swiss brands carry a lot more prestige as well, as their parts are assembled by the companies that the movements are going in to; not assembled by a Chinese factory then fitted to the cases provided by a different Chinese factory. The quality control issue is what arises when dealing with these parts, as inconsistences between factories and builders is great with reps. That is why we all look and rely on sellers here to provide with hand picked products; or for the other dealers who would be considered larger and do not have the time to pick pieces by hand having to deal with a very accurate and quality controlled factory/warehouse to purchase and drop ship their items for them. There was another post on here I saw recently that described the differences between all of the ETA movements, their qualities, and where they originate from and end up. Pretty much, it breaks down to the gens being a highly quality controlled ETA mechanism with COSC certification, a super duper seal from the seller, and different engraving on the back; with our ETA being sold to an "after" or "grey" market seller, who then assembles the products to their highest possible standards, but without any certification or guarantee. However, these movements are just as easily servicable as the gens and many watchsmiths have no problems at all.