If I was a pro, I'd hire him. He's bloody amazing.
On de Ligt he said recently:
"Reporting sick isn't an option for him, neither is an Arbitration case. I also advised him of this as it doesn't suit his nature and lifestyle."
There you go. If you see him slinging shit in the media, chances are his client asked him to. People should realise that before blaming him for some of his clients' digressions. Plenty of his clients never went through any transfer or contract drama. But if Paul Pogba asks him to do whatever it takes to get him to Madrid, he'll oblige, because that's what a great agent should do.
Clubs have the fans naturally on their side and with media briefs, official statements, club ambassadors and other mouthpieces know how to drive their agenda very well. It's only fair if players have somebody fighting their corner.
And to answer the question in the opening post: clubs will do business with him because he's hired by some of the best players around.