Why do Pokemon post game stories suck (aside from Johto’s, and even that doesn’t have much for story)? The more casual fans might stop after the credits so really they should be for the hardcore fans; it’s really a chance for Game Freak to step up their storytelling and give us some good lore. Instead we get Looker (arguably the best thing other than Johto’s and that says a lot), Team Rainbow Rocket (minor nostalgia dump with huge plot holes), and now Squidward and Sherbet (I refuse to ever use their real names).
Well, I should start by saying that I actually quite like Sword and Shield’s post-championship storyline in spite of Swordface and Shieldbutt (who are clearly not meant to be sympathetic characters), as detailed here (this question landed in my inbox before that entry was published). Their enmity towards Sonia for “changing history” actually does feel to me like a natural continuation of her story and an interesting perspective on the events of the main plot. So I sort of disagree with the premise of the question, which is a thing I do a lot. I also liked Fire Red and Leaf Green’s Sevii Islands storyline, and while I consider Looker himself a personal enemy who should be flung into outer space, I don’t actually have any issues with the Lumiose City storyline featuring Emma and Xerosic in X and Y. I have problems with the endgame stories that feel tacked onto a game that was already finished – the worst offenders in my view being Platinum’s Charon subplot (the main plot is about ancient mythology and seizing control of terrible cosmic powers to rewrite reality and change the nature of life, the universe and the soul; and then Charon, who’s been built up as this incredible genius for the entire game, just wants to blow up a volcano in order to extort lots of money from the people of Sinnoh) and the Team Rainbow Rocket saga of Ultra SMoon (which… well, we’ve been there). Also, frankly, even though this isn’t actually postgame material, I think the Ultra Recon Squad subplot counts as this too; it’s not actually bad in isolation but it doesn’t belong in the story it’s attached to. And, well, that’s kind of the answer to your question: because they’re tacked onto games that were already finished, in some cases (as I understand it – fact-check me on this) by different writers than the ones responsible for the original story. I’m inclined to blame the rampant corporate greed that dictates an annual release cycle for Pokémon, whether there’s actually a worthwhile game to be made or not – but then, I blame a lot of things on rampant corporate greed these days, so you can argue this is just my baseline.