Because he doesn't have near the natural ability of said players....quite simple really. Scholes and Iniesta are not under-achievers either, they are/have made the most of their careers. Cleverley is yet to do that although he is showing the signs that he might reach his potential.
No one's claiming that he WILL be as good or better than them (that's a LOT to ask for in all honesty), it's just silly to say he definitely won't, none of us knows what the future holds.
You can predict how far you think he'll go, but to say with certainty that there's no way he'll ever be that good is just wrong.