>>57135095Obtaining a Pokemon has pretty much ALWAYS been a metaphor for growing up, the trainer's journey is directly paralleled with the highly influential film Stand by Me in the very first game, when Red interacts with the downstairs TV and says it's "time for him to go, too." And that message of "becoming a trainer is comparable to growing up" continues even into Gen 9, where the whole point of the journey under the Paldea Academy, during the Treasure Hunt, is to give the students total autonomy so they can begin discovering who they are as individuals, and as such, what their aspirations and most desired wants in life are.
There's just no in-universe justification for why someone would wait until adulthood to get their very first Pokemon and take the first steps towards true autonomy.
Children in their literal single digits are eligible to receive a Pokemon and be thrown out into the world with nothing but their wits and that Pokemon to guide them, and with how Pokemon-obsessed the world is, someone who waits until adulthood to do that is very clearly a mentally damaged or traumatically haunted person that's waiting to become the next Cyrus.
Likewise, it doesn't work to take the "start the player off already having their starter with everything kicking off at a higher level" angle either, because that takes away from the emotional attachment of the player and goes against the "protagonists are canonical prodigies" detail. You're also forcing the game to start off in a more complex state (more diverse moves, higher stats, situations where abilities will come into play more often, etc.), which is bad for Pokemon's "every game could be someone's first video game" approach that makes it so goddamned accessible and easy to sell to the lowest common denominator.
Or, to put all of that in absolute brief to give you an answer, we don't get adult MCs because it goes against both the worldbuilding of Pokemon, and it means less money for them due to game design reasons.
