"Summon creature" spell gives you a particular creature. Such creatures exist in the world and are the same. The familiar is your friend: this type of being is not seen anywhere in the world alone. While you are getting experience, your familiar (or your power to summon one) becomes stronger. You are comparing stupid orc (or demon skeleton) to fellow created from your own capability.
On the other hand, more technically: with rising amount of high-level players Cipsoft realized that summons are too weak to be useful. Why don't they just not broaden the variety of monsters possible to summon? Maybe the didn't wanted to buff mages that way; probably found that summoning demons will make too big defference between calsses.