Consistent inaccuracy leads to drops of all sorts. It makes receivers have to expand their catch radius because they really aren't sure where the ball will be. Defensive backs drop balls a lot anyway.
Sure, these bounces and mistakes happen to all quarterbacks. But some much more than others based on how good of a ball they typically throw. If the receiver knows the ball will be in a certain tight area, then he's less likely to drop it, but if he turns knowing it could be in a much larger circle, the odds of adjusting quickly enough to catch it are lower.
Like a lot of quarterbacks today, Dak throws inconsistently and not in proper timing with routes.
He's not horrible, but he's not particularly good in that way either.