It's simple, Steph is the "system" KD is not. The system was in place before KD got there and it'll be in place when he leaves. He knows this and it's why he's willing to leave.
Everything they do is structured around what Steph can and can't do, not KD. So when you take Steph out of the equation they become just a good team.
Just look at how they run plays with Steph vs KD. Everybody is moving, Steph is coming off a stagger and Klay is on the weak side coming off a flare, next thing you know Looney slipped the screen for a wide open layup from the top of the key by Draymond.
When it's just KD they don't get everybody involved like that as often. And that's not a detriment to him, Steph is just really that fucking good. And honestly people forgot that.