Because Furor said so. I play a ton of Kat and personally I'm not too big a fan of it. I've gotten it myself. If the game goes on long enough you will have it fully farmed, and if you go AD you can actually get it there at a decent (for Kat) rate with BB farming and kills. Still, not a great item. That said, I DO like to get a regrowth pendant to start as Kat, and that builds into Warmogs. But for a giant health item, Frozen Mallet is better. You need at least one big survivability item on Kat in case you get targeted (which every competent team will do) during your ult.