Because they would have went out of their way to future proof the models on 3DS, tanking the framerate, only to throw them out the moment a new generation comes along? Let's Go has the same models, as has been data mined, and they look the same.
If the models were remade from scratch, then why do they look the same with minimal improvement!? And no, a texture tweak does not mean the entire model was remade.
The Dynamax models just look like scaled up normal models, so I'm pretty sure that's bull too.
I'm starting to feel like this is just a ploy to trap pokemon in HOME so people will pay to not lose them.