Kaido was not born the strongest in the world, he became, in short, it would be the minimum for the navy to capture Kaido alone, he challenged them alone without a crew with nothing, KAIDO did not have the structure he has today, in members and military power.
Not to mention that he was captured +1 time, which means he could escape whenever he wants, I believe that KAIDO himself allowed himself to be studied, nor did he himself probably know why he was so strong.
Well and the world has turned and now it is vegapunk who would serve kaido: