Because there's still s*** like Sniper Riki and PA. And because people just don't want to end the f***ing game when got mid rax at 25 mins and just farm and wastes the guy who gets put into a team that instapick PA and Sniper and Riki's time and just want to find another f***ing match.
Obvious, yes. Happens a lot, not really. Which is why it is normally successful in pubs. The enemy team facepalms for missing it completely. And yet it still happens, either because the ones who are aware of it don't want to deal with it, or because they're not expecting it to happen on that game.