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I have a little headcanon about evolution through trading.
I think that these pokemon previously evolved in a normal way i.e. level up, but had found that they did not need to evolve into these Pokémon as they were already suited for their climate/background/whatever. This would explain why pokemon that DO evolve by trading often look pretty tough already, because to them, that is their final form. Through generations upon generations pokemon like Graveller or Rhydon have evolved not to need to evolve again, their bodies have stopped producing whatever energy causes them to evolve - it is only when they are introduced to these 'trading machines' they are exposed to this energy once more and forced to evolve into an unnecessary final stage.
This theory would explain why Pokémon like Scyther don't increase their base-stats upon evolution, and also why finding fully-evolved trade evolution pokemon in the wild is so rare - they're dying out.
