Good point.
But does this mean that, if nobody in Kanto mentions Kyogre, it's not true that it really made the sea?
So Kyogre and Groudon's powers are most likely just imaginary.
if you want to get scientific about it.....
Kyogre and Groudon are a physical depiction of the constant changes in the land and sea.
Erosion makes oceans seem bigger at the expense of land, and land has constant eruptions and tectonic movements that alter the land masses. In my opinion, because of their powers and abillities, people (of the Hoenn region) feel that they are godlike.
Groudon, like I said, represents changes in landmass, his abillity causes Sunny Day to go into effect, and we all know that hot sunlight evaporates water. Its moveset and abillities go to show that it can impact the land... but even a Diglett impacts the changes in land with the tunnels it digs...
Kyogre represents changes in oceanfront, and his abillity causes it to Rain constantly. We all know how rain redistrubutes moisture and creates puddles in the land. Its moveset even goes to show that it impacts changes in the water, but all it really does in terms of the water is shift it around... but even a Magikarp shifts the water when it splashes.
Groudon and Kyogre are a check and balance to eachother, and as a result, depict the cycle of land mass changes and oceanfront changes.
Rayquaza, I know I have to mention him to get this to work, is a powerful depiction of the constantness of SKY.... not matter how much it rains, volcanic eruptions, earthquakes... no matter how these things change the surface, the SKY still looks the same. Rayquaza is a powerful pokemon that depicts the constant steadiness of the sky. When it battled with Deoxys, it was battling an extraterrestial being, and that is similar to how our planet's atmosphere burns down meteors, and even deflects them. Its moveset revolves around elemental aspects of the sky. And besides... even if it is raining, if one was to go further up, ABOVE the clouds, the skies are clear.
I mean, in the literal sense... what are Groudon, Kyogre and Rayquaza? They are an Armadillo, a Whale, and a Chinese Dragon (the snake like, flying appearance versus the Medieval Europe Lizard like appearance similar to Salamence). These creatures themselves are capable of things out of the typical. I know that comparing a Chinese Dragon in terms of realism fails because it is a mythological creature, but an Armadillo... it's a very durable animal that can survive in various conditions, and a Whale, well... it's a mammal that lives at sea, it is vast in terms of its appetite, and it constatly travels all over the world.
Mythology and Religion create explanations to things that we don't quite understand until science comes into effect and these pokes are so powerful that the ancestors felt they were godlike.... and we definitely saw that when they battled, the way it impacted that part of the world.
In the real world, some religions feel that certain animals are depictions of things, sacred, and possess holy abillities..... like Cats in Egypt being guardians of the underworld... Cows possessing the womb of life... and so on... and yet, once again, these were regional beliefs.... in one part of the world, we're slicing up cows to make meats, in another part, we pray to them... hmm....