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I'll tell you why anon.
Pokemon is all about player agency - the whole point of Pokemon is that YOU get to pick your favourites, YOU get to choose your team build, YOU get to choose their movesets, YOU get to choose what balls to catch them in, etc. etc.
Therefore, in Pokemon, more freedom is always good if executed correctly.
Currently, a typical Pokemon adventure goes like
start town > first route > second town > forest > third town > second route > cave > fourth town etc. etc. It's too predictable in the year 2022.
Now imagine a BoTW-size open world where you have to explore and find the locations of the Pokemon you want - with rarer Pokemon requiring specific tasks to be completed before they appear (for example, breaking a specific part of a cave with a pickaxe to go in deeper). Gyms that aren't just in a town, but hidden up a mountain or in a random underground dungeon in the middle of a dense forest. Or a giant desert that seems endless and lost, but there are rumours from NPCs in the towns of a legendary that lies deep beneath the desert soil that hasn't been seen for hundreds of years, and you need to figure out how to bait him out.
See? The imagination can run completely wild on the open world concept. If done correctly, it would be objectively better than any Pokemon game to date.