This may seem like a very simple explanation, but the difference is that the league is made up mostly of guys who can "make passes" and a select few who "make plays."
When you watch Rattler play, he shows flashes that he is a guy who will go out and "make plays"...at this level, every QB can stand in the pocket and deliver passes on time with pretty good accuracy. The thing that makes guys like Mahomes, Lamar, Josh Allen different than guys like Justin Hebert and Gino Smith is that they are able to generate positive plays off script. When guys are covered up and the pocket collapses, is it a throw away or 6 yard loss? Or is it an escape with eyes up looking for an outlet or scramble?
No telling who Rattler really is at the NFL level, but those folks on here talking about how he could be our future franchise QB are looking at (1) arm strength, (2) escapability, and (3) off script plays. If you have all of those things, you have a chance to be special in the NFL.