Caesar provided Doflamingo with the knowledge on how to produce SMILES, and in turn Doflamingo mass produces them inside his factories. At the moment Kaido has no need for Caesar, as Doflamingo is doing all the work for him. But if something was to happen like say, the factory gets destroyed, along with the production of SMILES, then Kaido no longer has his steady supply of SMILES coming in, and he's going to need Caesar directly in order to start reproducing them. It's like the Big Mom situation, although she loves sweets, she doesn't rely on her own crew and manpower to make them, she relies on the towns that are under her protection. Another reason could be because, being considered an underling of a Shichibukai grants Caesar exemption from the laws of the World Government, which wouldn't be the case with Kaido. Leaving Caesar alone on an island to carry out his experiments would be a huge risk, especially if the World Government knows he producing weapons for war. However since Caesar works under Doflamingo they can't touch him.