Defense of the Ancients 2

The thing to realize is, Dota2 champions are much more powerful than League champions, and in this game, carries carry a LOT harder. I'm serious. In League, even if you're super fed, you still won't be taking down the enemy team without your team to back you up (unless you're playing against bots), but in Dota2 it is not only common but expected for a team's carry to be able to kill two or three members of the enemy team entirely on his own.

Windrunner doesn't have an adequate League equivalent to really be compared to, but the thing about her skillset that makers her unfit for the role of carrying is that she simply doesn't do enough damage. For starters, she's an Intelligence hero that doesn't have any nukes, so stacking Intelligence carry items isn't going to do much more for than give you mana that you don't really need. It will give her damage, of course, but what is she going to do with it? Her ult causes her to deal reduced damage to the target, and if she changes targets she loses the attack speed bonus. Her kit is designed around position-based support, hit-and-run tactics, and in the case of her ult, tower killing. But if you really want to know why she's not a carry, give her 7,000 gold and give an actual carry 7,000 gold and see who wins the fight. 10 times out of 10, Windrunner will lose that fight.

Windrunner qualifies as a semi-carry because a competent player with a careful item build can carry a team in the right situations--but there's a big gap between the abilities of a semi-carry and a carry. And don't even get me started on hard carries, lol