Well first and foremost Edo Tensei.
After that, while yes it takes prep... It isn't this forever long process. The snakes worked like shadow clones by distributing chakra meaning he doesn't know which one to hit... Plus with skin shedding most attacks except the totsuka can be dodged.
While he has Izanami, a living Itachi would have died multiple times before activation... Even placed in a position to be over written if not for help.
Kabuto has absorbed DNA from Suigetsu, Karin, Juugo, and the Sound 4... I think even Kimmimaru. He's invulnerable to physical attacks due to hydrification. Heals fast due to Karin, Infinite Sage Mode, huge chakra level (something Itachi lacks), sound genjutsu against Susanoo, Jirobo's move to make a cave of he chooses, and the list continues.
Simply put he's a walking collection of people... He's stronger than Itachi, and even if he wasn't he can outlast Itachi.
Edit: sorry for the really general explanation... Simply on my phone...