A lot of people have already posted what I'm about to, but really it's about the experience.
I'll use ORAS as the example since that seems to be where the crux of the concern is in the OP. Yes, we received MOST of the Pokémon at this point, with the exception of some of those missing event legendaries and Volcanion. But the there's abilities that we still haven't seen yet, such as Kalos acceptable Kabutos with Weak Armor. I know they can be received from the 5th Gen carts then bred-out, but for VGC purposes they still aren't valid Kalos region Pokémon because the hidden ability hasn't been released in XY or ORAS. Along those lines, we ALWAYS get new moves on existing Pokémon with each installment, whether it's via level-up, breeding, event or move tutors. So there's that for the competitive players.
For the aesthetics, new carts means new animations. In ORAS, most Pokémon now have an entrance animation they did not process before in XY. I also believe some Pokémon received additional animations, some even received slight changes in their polygons. New move animations are always something to look forward to. My favorite analogy is Outrage in B2W2 compared to it's predecessor. Simply sick!
And then there's story. We already know Zygarde is getting the exclusive treatment, and we know there's something about that special Floette we've yet to receive, Volcanion is still MIA, so naturally there's going to be additional story arcs in the third entry. Then there's the lack of story concerning the Mega Stones, which many people are still hoping get a fuller explanation in the third installment.
So there's a multitude of reasons. You can pick any of them, all of them, or none of them. But really it's just going to be that; your personal experience.