Why do softball umpires take 20 seconds after the pitch to call a strike?
At least they are consistent. They all wait til after the ball is fully in the glove, take a look at it for a few more seconds, then begin the slow process of raising the arm and grunting some version of “Haeyaaahhh”.
If you have to wait that long to make a call, it’s probably not a strike. There’s no reason not to begin making the call when the ball hits the glove. Otherwise you are allowing yourself to be biased by the catcher moving the glove to frame the pitch after the fact.