Because if you become a Sith, you are a Sith. You would be a former Jedi then. Its like the next level. Shooting lightning, ruling the galaxy or overthrowing your master is what any darksider can/would do if its useful for him, its just an evil act. Being a Sith is more than that, its a whole culture, and like I said I dont see Dooku being 100% into it, judging most of his behavior. Also he turned to the dark side after Qui Gons death, means there are only few years where Palpatine taught him stuff and probably not even that much, since Anakin was his favorite and Dooku was already old.