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2 reasons:
1. to keep gameplay and cutscenes 'consistent'
2. to give players more of a reason to check out events. Most players probably won't give a shit about special moveset shitmons that end up being worse than their regular movesets anyway, but they will care about their sparkly legendaries.
Of course, both are retarded. Shiny chances are low enough that probably only a few hundred to a thousand people will encounter a shiny legendary on their first playthrough, and it's not like the cutscenes are interesting enough for continuity to matter. And for the latter, fuck modern events. They're boring grab bags that you get with your Gamestop purchase if you have shit internet or you hop on early in the game's life and you get free crap that trivialises the game.