The difference is that the cards don't actually exist. They were able to appear in the TV world because reality was distorted that way, but the cards are a representation of one's soul, making it impossible to materialize in the real world (Since you would have to rip your soul out). An Evoker, on the other hand, allows one to summon their persona by simulating strong enough emotions that they're forced to summon their Persona.

This might be completely wrong, that's just how I saw it.