Because all of them together still won't amount to a Kaido. Until you think CP0 = 1 Yonko.
CP0 is not the strongest fighters of the marines, but rather the best assassins as their tasks involve great deal of stealth and secrecy.
I differ on this. I think that the CP0 will have at least one admiral level fighter, probably the leader. Stealth and secrecy isn't the only think they need to be good at. Otherwise, CP9 wouldn't be on the level of fighters that they were. As said by the Gorosei, the Marines are just the outward face of the WG.
CP0 has been hyped as the 'strongest intelligence organisation' and it's not because of their stealth and everything, in Japanese, it's the same as Mihawk and WB's titles. To work directly under the nobles' command, they need to be very strong. They're literally the shield of the Celestial Dragons. But let's see how it happens.