As was previously mentioned,I think its ultimately a partnership that only the humans can create
due to their chakra, perhaps. Perhaps its the human blood that allows for such a d eal to be forged. After all, we don't see the summons signing the contracts themselves. But that brings up the question of why they would need such a thing to begin with? We've seen the frogs arrive at Konoha on their own without being summoned forth, so perhaps 'allowing' you to summon them is a gift of sorts. "Call on me if you need me."...at least for the frogs and Kakashi's nin-dogs (though they seem subservient).