Gardevoir's egg group is amorphous, despite it being so human like. Why is this?

Well, I suspect the Gardevoir line may not be entirely organic

Apparently, Gardevoir are based on a type of Japanese paper doll called anesama ningyou. The Gardevoir line - or at least Ralts - are affected by emotions. Shuppet feeds purely on negative emotions

So if Gardevoir are dolls, and Bannette are abandoned dolls...

...Then any chance that if a member of the Gardevoir line gets abandoned, they go nuts and start feeding off of their own negative emotions, their own feelings of abandonment, until they get corrupted and turn into a member of the Bannette line?

Only issue with the theory is that, well, wouldn't Shuppet evolve from Ralts then?

That would require a low happiness evolution. And GameFreak makes the games to encourage friendship and the such. So why would they go against that message by putting in a Pokemon that requires abuse to get?

So instead they are two separate lines