You can't be serious. By this logic, if you are a Safety, you should easily play CB or in the slot. With a few minor exceptions, that is certainly NOT the case. Likewise, why are so many defensive backs poor catchers of the ball? It's all the same, right?
As a punt returner, you need to catch the ball cleanly, in traffic, EVERY single time, and that's before you advance the ball one yard. Anyone who has shagged punts in their backyard, let alone during a game, will understand it is very difficult to do competently and without error. And there's a good reason why VERY few pro football players are good enough to do it well in a game situation.
That's what's made Shakir so valuable. With a great deal of confidence, you can count on him to catch the ball and hand it to the offense. The problem is, he's never consistently demonstrated an ability to get decent return yards. Why? Because it's so hard to do everything well.