This is because Mew can only be acquired by collecting the original 150 Pokemon and completing the Kanto Pokedex, meaning you’ll have to open a serious amount of packs in order to even get close ...
After this event, there still might be gaps in your Pokedex thanks to legendaries or regionals or the fact that you still can’t snag a Mewtwo invite or catch a Ditto in order to find Mew.
If you have, say, a Pikachu from Kanto registered in Home, it won’t count towards your progress in the Pokédex for Paldea, Kitakami, and Blueberry Academy. You will have to actually go and ...