Because act 3 tragedy, they would have to reprepare themselves and then go for fights..., there's no time to prepare Kaido is about to land on Flower Capital
Except if Marines come or something/one else
The tragedy will be the landing of Onigashima on Flower Capital, Kaido will defeat Luffy, people will be enslaved and people will see that orochi is dead and they lived a lie all this time, they will call for Luffy and he will come back with help (either Marines or Grand Fleet)
LUFFY HAS TO DEFEAT KAIDO IN FRONT OF EVERYONE