>>469122591) Because it's a game with a mostly child audience in mind, so self-insertion
2) Because it makes more sense for a child to just start on his journey than an adult who probably would have gotten his first Pokémon years ago
3) It also conveys a stronger sense of starting as a newbie and building oneself up to fight against big bad adults, and that's why they can shower you with tutorials and everyone can explain things to the player ad nauseam because the player is a child in game
4) Because of tradition, having made child protagonists for over 20 years and basing off Red and Ash
5) Because they're cute and fit in with the marketing of Pokémon better than an adult would
