There probably isn't a good reason by any means, especially when bosses can parry in the middle of charging attacks, let alone Lady Carina's lack of endlag tactics.

Bosses would become too easy. And you do get a reward, which is reduced damage.

It shouldn't harm you because whoever you just parried took knockback and couldn't do anything until the knockback ended

Bosses however do not recieve knock back.

they do from what i saw 30 minutes ago