Imo that is to create mystery as per usual if they raced for it and in the end let's say Kaido and BM took an equal part and BB a lesser then everybody would have legitemely said that Kaido and BM are equals and BB weaker yet he make BB take everything to make him become a legit yonko (unlike Luffy) who competes with the other ones (and leave the power relations as obscure as possible).