Yes cause they were strong and want them in his army. The Kappa was in jail for like 13 years. He should know the sccabards ain't changing. Its no tension in this story.
Like maybe Kaido is just very arrogant but IT'S STILL WITHIN CHARACTER. He fully believes he can turn anyone and even after decades of trying.
So your saltiness here isn't about kaido acting out of character. It's him not doing what you wanted