Because it’s the flaw of the laws. When does the catch start and end? He had it in 100% in control in his hands, thats the catch, ball is dead. He throws it up in the air for the celebration, hits the floor after the celebration, but it is a catch.
The whole ‘in control of the body’ phrasing is nebulous nonsense, equally as nebulous as ‘fingers under the ball’, which they were, not a lot of finger mind, but they were still under the ball. Everyone that isn’t from England thinks it was a terrible umpire decision. Whatever, it’s passed, I’m sure the next non-catch will be given as a wicket to even things out again.