Because Oda is just an outsider of his own manga. You have to view things as how Oda created them, not to how fanbase wants it.
The bounty is not given by Oda... I mean, it is given by the World Goverment with the info they have.
Law and Kidd should just get the split bounty of Big Mom... which is 2.17 billion, even lower than Teach's first Yonko Bounty. Yet, they still didnt beat her, so their real threat power should be 2 billion at best just as Crocodile.
While Luffy indeed damaged Kaido for like... the 80%? So yeah, Luffy should have 3 billion and Law&Kidd 2 billion.
But since they were the captains... and the info is limited, they get equal potrayal.