The backstory of the Pokémon universe has never really been explained, but some people think a history of devastating warfare and thousands of lost lives could explain a lot of things about the Kanto region.
According to a Creepypasta theory, the lack of adults, the amount of orphans and the children's' fixation on violence can be explained by an encompassing war that decimated the population. The theory reads:
You have no father, and your best friend is an orphan.
Children are going to a journey around the world.
Your mother accepts you as the man of the house, making your own decisions.
Old man warns you of dangers around the corner.
Most people you meet are children and/or gym leaders.
Most adult men are either very old, or have a job related to the military or organized crime.
Hospitals and gyms are everywhere, but no forms of entertainment such as movie theaters.
“Hey kid! What do you think you’re doing here? You won’t live long in combat!
That’s for sure! I tell you what kid, electric Pokèmon saved me during the war!”
You now realize that there was a war in Kanto.
Your father died, and your rival lost his parents.
You are among the first generation of people to live in peace at post-war time.
It certainly explains a lot of the dark touches that you may or may not have noticed in the world of Pokémon, but if you're not totally convinced, maybe this new video by PBS Idea Channel might be able to sway you.
When you think about it, this also makes your quest to fill up the Pokédex seem so much more relevant. If Pokémon have been around since ancient times, they must have been documented before, so it's entirely possible it's your job to see which species managed to survive the ravishes of war.