Part of it is that they're good receivers, but there's more to it than that.
Coaching is a part of it - McCarthy has become a mad scientist at designing plays that create space for the receivers. And, when you have that many good receivers on the field, there's almost always going to be someone open, because you can't double everyone. You can double Jennings and Nelson, hope a safety or linebacker can pick up Driver in the slot, and put another safety on Jones, but then who's left to cover Finley? One of them is always going to be at least somewhat open.
But more than anything, I think it's just Rodgers. He makes guys open who'd be covered if anyone else was playing quarterback. How many times have you seen a DB have absolutely textbook coverage on a receiver, and Rodgers hits him anyway? He only needs a window the width of the ball, and no matter how well you're covering a man, there's always a spot where he can reach the ball but you can't. Rodgers just throws to that spot. That's it. That's how these guys are open. It's Rodgers more than it's them.