Because each individual costume must be levelled.
What you summon in the Portal is a costume PATTERN. You then use that pattern on each unique copy of the hero (thus becoming an actual costume). You then level the costume up to get the full benefits.
This is covered in the following three threads:
-
Costume FAQ’s in Q-2
Hero Costume FAQs! Please read for answers to most questions before posting new questions -
Costume Levelling Guide
Costume Level Up - Experience and Materials Required + Experience Gained via Feeding -
Common mistakes to avoid
Avoid The Following Mistakes With Your Costumes