1. She isn't a tank, she isn't a fighter, she's an assassin, she is squishier than an ADC for most of the game and dies really easily when she's camped or alone.(by this i mean she only brings damage to teamfights, just like an ADC which means she needs someone to tank for her, and CC for her. If she doesn't have that, she's the same as telling your squishy jinx to go 1v5, regardless of how fed she is, she'll probably die)
2. She isn't meant to engage for you, not on a 5v5 teamfight, it's not her job, she'll just die and do nothing, then you'll flame her for feeding or split pushing when you haven't been trying to help her. She's a teamfight assassin, if she doesn't have a team, she's nothing.
3. For the love of all that is holy, pick someone with engage, anyone, katarina cannot engage for you, she can't keep going in alone because she'll keep feeding, you need to engage for her so she can finish the fight quickly.
~Your neighborhood Katarina main who can't win a game right now because no one wants to engage.