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They still need to program them in, make sure everything is working as intended and smoothly, and when a new engine is being used, they have to also make sure they're compatible first. They have the base models and animations, but they were programmed for the 3DS engine. I saw someone use MHW as an example of remaking the models, but all they did was use the old ones as a base, even using the older animations still. However, they couldn't get the Leviathan monsters in as the skeleton was incompatible with the new engine. There's a lot more involved than people think. Future proof=/=easy import. They made the models with high polygons to use as a base and rework them if needed, like higher quality textures or shaders.
Let's Go has the data of all Pokemon, yes, but its because it uses a very heavily modified Gen 7 engine. Any Pokemon game, or even a game franchise in general, using the same game engine is going to generally contain the older games items as leftover data. As another example, Generations uses the Hedgehog Engine and contains leftover data from Unleashed, like UI elements and the shaders.