What is the reason it feels like most if not all successful watches were designed by the exact same guy?

Now, the reason it feels so, is that it is factual like this -- the majority of blockbuster designs in watches were made by Gerald Genta and no other.

And what is the secret of these designs, that makes these watches so successful, you ask? Now, it is the combination of round and angled that is so artfully combined in his concepts. The Ying & Yang in design.

The Royal Oak (1972) his most relevant strike is octagonal and this is essentially the combination of a square and a circle in the very same form -- even the screws were faceted / angled. But the dial remains round to not break this classic design of a watch and to have enough Yang in this watch. And this concept is used widely -- either for the Nautilus, the JLC Master Mariner, the IWC Ingenieur, the IWC Yacht Club II or his own brand.

The masculine squared and the feminine round allows almost everybody to find his or her favored design elements in these watches and makes them much more widely accepted timepieces than others that hold just one of both characteristics. Besides the fact that masculine and feminine go usually good together in some aspects -- one obviously being watches. The watch Ying & Yang.

Addendum: Even the master of watch design needed some time to fully understand, refine and bring this recipe to perfection. Early trials were already in this direction but lacked the perfect combination and proportion of the key "Round and Angled" (RaA) or "Round but not Round" (RbnR) -- for example the Patek Ellipse, the Omega C-Shaped, these are not perfectly round and tend towards this recipe but they lack the right proportion of "Angled".

And the Rolex King Midas on the other hand side is probably one of the most angled watches he ever made, but lacking the feminine round, lacking the Yang while having a lot of Ying.