I mean, it's a question of realism vs gameplay.
Walking across an entire region in less than 2h without food or water makes absolutely no sense, but forcing you to eat supplies and stop to rest wouldn't be conducive to good gameplay (at least not in Pokemon), so we just pretend that doesn't happen.
Adding more people in RBY (assuming they did have the cartridge space for it, which they most certainly did not) would be more or less the same thing, without bigger maps and custom dialogue they'd just be clutter - like the random people you walk by daily but never talk to, realistic but not a fun experience.
It's no surprise that the same link you posted said that Kanto (the first region and the one done with the least amount of cartridge space) has consistently been the smallest region by population (even after getting a small boost in Generation II).
And it's also no surprise that other artforms like the manga and anime have bigger regions with more "extras" and characters: they're needed (and inexpensive) to throw in there.
And to be fair, even other, much newer games have unrealistic populations, for much the same reasons:
The biggest city in Skyrim has 74 people, which is about the amount of people in Celadon, and about 8 to 4 Viking households if a quick Google Search is right.
Stardew Valley's 28 villagers and Animal Crossing's 20 ish villagers (counting the player, Isabelle, Nook and the Nooklings, etc.) are also nowhere near close to the population of a small village (500 to 2500)
And so on.