Also because Harry is, in fact, a garbage-tier Wizard. He's got the talent for sure, but he's never bothered to develop it in any meaningful sense, instead relying on his big gas tank as a crutch.
Elaine (Half-trained Wizard) is stated to have more skill then him, Carlos (several years his Jr) has more skill then him. His apprentice (who he trained) has more skill then him.
Harry has spent the last decade or so dicking around while everone else caught up to and then surpassed him in skill.
He's like, the Wizard equivalent of "Peaked in High-School" these days.