Much like Equipment, Auras, and traditional Vehicles, they risk clogging your hand if you draw too many without enough creatures to station them
If you are playing an aggressive deck with green you will play Goyf, but your deck wouldn't really suffer at all should you replace him with some other two drop dork
Though it enters the battlefield tapped you can use Arid Archway to add two generic mana to your mana pool which is pretty good on its own
During the opponents turn, it can protect itself from card effects
But the only way to differentiate them at that point is to make really complicated logic, so you constantly need to make more complicated stuff. Creatures knows Pokmon must, at some level, be something any child can learn quickly, so its opted to bump up the numbers instead of introducing too many new ideas