Because the one rule when fighting against Eldar is to never stop your transport if your the driver. As soon as it has i can almost guarantee It's dead. Haywire bombs on Hawks are a way for them to make you stop, which is usually what a decent Eldar AV will do. A haywire hawk spotting for a WS of two dragons. You think it's bad on a rhino? Try having a transport you can't turn on the spot, or hide behind cover though that's AV in general not just Eldar. At least it's not Orks OP anymore.