You just don't get it, his agi gain is not that good, his only ability to scale is the stun. With the same amount of items, you can't even fight against other carries.
The stun does not hit magic immune target which most carry will be. You against other carries he does not have any scaling ability. He's also very soft, getting hit 2-3 by other carry can get you killed.
First of all, you don't know what you're talking about. His mini-stun DOES go through magic immunity and he destroys almost every carry late game because they can't even get close enough to do damage.