Big Mom never should have made it to the roof. Kaido and Big Mom alone are still too much for the SN to handle, and putting them both together was just overkill. So Oda has to have them just stand there and take attacks, otherwise the War would have been over already. I get the whole "Strength in numbers" thing Oda is going for, but that only works when the majority are at least somewhat comparable to those they are fighting. As it stands though, in a 1 v 1 fight Kaido and Big Mom steamroll anyone on that roof. Hard to get into the fight when you can't suspend your disbelief to that degree. Oda must have realized this, which is why everything is getting off paneled, and he's decided to split the Yonko up.