>>40818606GF's business strategy is to blame. They can't churn out main series games on a yearly basis with every single Pokemon and derivative of that Pokemon in the game. It's well over a thousand when you take into account the different kinds of Unown, Trainer Hat Pikachu, Furfrou's hairstyles, etc.
At this point, the games exist to sell merch. It has been this way since 2013 and shows no signs of letting up. Why put an exorbitant amount of effort into something that exists to prop up the sales of figures, plushes, apparel, cards, etc.?