Because Kaido isn't the world strongest
Every time we head into the villain he seems to be at the top of the world then we find out that there's more after him; that's literally shonen 101
for the entire from Punk Hazarad to Dressrosa to Zou to WCI to Wano to Onigashima they kept talking up Kaido and when time came to defeat him, Kaido started hinting that there's more with shanks in his top 5 and haki transcend all then Shanks showing up and doing crazy feats Kaido couldn't do
Obviously there will be a lot of powercreep in the final saga
We started it with Shanks but you can bet Sakazuki, Teach , Mihawk, Dragon and everyone saved up for the final stretch are gonna be more powerful and broken than the ones who came before them