Because being able to shred tanks without needing your ult is incredibly useful, and it's great as a third item.
If the game isn't over by the time you get it, you need it, which is the logic for buying it.
And I'm getting pretty gud. S- even tho we lost.
The issue is it's dependent on you getting a knock up. If you have the tank in your face right now, you won't necessarily have the time to get the 3 q's for a knockup. And relying on your team for a knockup is not exactly a game plan.