Would people rather see Luffy defeat Kaido 1v1 like this or Supernova captains defeat him 3v1
Second lets say for argument sake Luffy beats Kaido 1v1... then what's really next? Who is stronger than Kaido
Shanks might be the strongest after all and BB is gonna surpass Kaido too soon if he is not already above that level.
it is just Shanks is a peaceful man. he wants peace while he is waiting for the true pirate king to come he knows what will happen in the future. I assume if he wanted he would have already defeated Kaido and BM.
Kaido and BM are violent and are willing to go after one piece any moment if they could. it is just they know that they cannot defeat Shanks.
BM admitted herself that if she had the giants power she would be able to defeat Shanks, which means she doesn't have the power right now to do so. the same apply for Kaido he was amassing tough soldiers, even wanted to force the Super Nova to work under him. they need more power to challenge Shanks.
Shanks might have a lower bounty than Kaido and BM but at the end of the day he is peacful and have even some sort of a relationship with the WG. so they don't consider him to be the danger for them.