Because Griever was one of the major elements in the story. And if Ultimecia summons Griever, you (as the player) are going to think, "Oh my gosh, she just summoned Griever."

If she summoned Norton (who hears a Who), or Robert, or Humbert, or Delilah or Nofundo, there just wouldn't be that shock. It would just be some massively random GF!