I always thought that Pokemon gyms only use one type because in-universe that’s just easier to maintain. It’s easy for us to look in at Pokemon gyms and say they’re dumb for not diversifying but the implication of every game is that we are the best and most skilled trainer in the region that becomes the champion and maybe fights a couple gods and deities, of course we think it’s easy. But like someone who has only owned a bird would probably not already have the same knowledge and tools to take care of a dog. Same with Pokemon. Someone who takes care of water types would have an easier time with another water type rather than something like a fire type. That pool in my backyard might be amazing for my Squirtle’s enrichment, but if a try to get Charmander, it becomes a hazard. I may love my Sylveon but I need to make sure she doesn’t play too rough with my Hydreigon. Not to mention, the amount of care 6 different animals need already. Realistically, you can’t just keep them in a PC or ball all the time. You need to take them for walks, feed them, give them water, some sun for the grass types, maybe some souls of sinners for the ghost types, perhaps a mountain to topple for Tyranitar. But since we’re just playing a game we never have to think of any of that.
