>>56970800They make sense in-universe. Trainers actually have to raise and care for their Pokemon like pet owners do IRL, and that's a shit ton of maintenance that we never see or have to do ourselves because it'd drag the game down massively. Sticking to a single type or species with notable similarities makes it easier to generalize your care routines, rather than having to fulltime invest in learning how to care for multiple completely different creatures. This is also why most trainers struggle to have more than three Pokemon, because that's increasing your maintenance load.
You have to be a canonical prodigy like the player character or someone who's devoted their entire life to being a top-tier trainer like Ace Trainers, Veterans and Champions to actually achieve the type of effortless diversity and "full team with plenty of Pokemon in the boxes waiting to be rotated in and out at a moment's notice" that we're accustomed to, and most trainers are just hobbyists who've found a way to integrate Pokemon into their passions, such as Hikers, Fishers, Black Belts, so on and so forth.
