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Fundamentally, it's because the people who design the Pokemon don't decide their stats, abilities and move pools.
One of the most important things for a Shillmon that didn't come from Gen 1 is POWER. The player has to feel that they are STRONG because they have said Shillmon. Lucario and Greninja's are the easiest examples of this, because they follow the "Glass Canon" forumla. People don't care think a Pokemon weak if it faints, they only remember how easily they took out other Pokemon.
Zoroak COULD have worked but it's gimmick meant to allow it to be overpowering was basically destroyed by Team Preview. It was clearly designed before they'd decided on Team Preview and since the main Pokemon members don't really "care" about battle viability, they likely thought having a "cool" ability is enough.
Silvally's failing is that likely someone at Gamefreak is thinking "it's a fake Arceus so it MUST be worse than Arceus" so that person who was likely in charge of Silvally's stats, movepool, and ability likely kept hitting it with the nerf stick, because they kept thinking "It's a fake Arceus so it needs to not be able to do things Arceus can"
This is the weakness of every Pokemon, because the designers aren't in charge of what they can do.