We don't lol.
Picca was not close at all to Zoro so why you suggest we saw his max Haki usage?
Enma is the sword that scarred Kaido not the only sword able to scarred Kaido .
Oden simply went beyond other couldn't and I don't see the issue CoA and swordmanship has been hyped a lot in the time we been in Wano.
Zoro says he can't cut Pica from distance like Mihawk could, and needed to be launched near Pica so his slashes can completely cut Pica Golem,
I didn't say Enma is the only sword able to cut Kaidou, Top tiers have vast amount of Haki, they don't need such sword.
But characters like Oden who are a tier below Kaidou, can use Enma to make one of his strongest slashes by using all his haki, to perma scar Kaidou. That's why his other name sword, that is also of great Quality didn't perma scar Kaidou.