Sigh.
Would that you have played Kat before coming here, not just played against her. Let me lay it all bare for ya (note: this will come from a long term Kat main):
There's a conglomerate of reasons for Kat's defensive stats, all boiling down to one: she's slow.
- "slow?! You don't know what slow mea--"
Yes, she's slow. While she has access to ~1 shunpo/second up to 3 shunpos per combo, it doesn't change the fact that setting up proper trading ground takes a while, leaving her absolutely exposed for the duration. It's over 1 second since shotting Q before you can pick up the dagger - it's more than enough time for the enemy to reposition or lay some AoE/trap where your dagger's gonna land. Compare that to
,
, and even to
, who might not have access to instant burst that previous 2 champions mentioned do, but has a reliable way of repositioning. Kat is predictable as daggers are nothing more than bread scraps left behind, indicating where she will have to go.
Then there go shunpo nerfs; Kat has to get 3 ranks in E early just to be able to somewhat abuse her mobility. This leaves her with no reliable source of poke or waveclear (she has to abuse daggers to farm effectively, and that also leaves her exposed). Either that, or higher Q poke but no trading potential.
This, paired with her lack of innate sustain, hurts terribly. She's actually outscaled by many champions pre-6, and even after - unless she managed to get ahead already.
Gunblade + Ravenous Hunter rune fix the aforementioned issue somewhat, but both take time to activate. Defensive stats are meant to allow Kat to survive the laning phase in spite of her ridiculously risky farming sequence.
TL;DR Kat is much slower than most assassins in the game, second only maybe to
and
. Those have either safe, reliable trading tools or innate sustain, though. Kat makes up for the lack of those with higher base defensive stats.
If you want to talk about the mid/late game though, be my guest. I'll gladly share my insight into why people get wrecked by random Katas so easily.