Kabuto's medical ninjutsu knowledge was ridiculously important to all of Orochimaru's research and shit, so the idea that Orochimaru must have every bit of information and the resultant strategic advantages is just not right. In addition, Orochimaru hasn't exactly gone back to base to collect any of Kabuto's shit either.
Second, the reason that Orochimaru does not have a body capable of being a full sage is the same reason Kabuto needed Juugo's DNA to manage it: they are not true sages
. Note how Hashirama, Jiraiya, and Naruto all lack the inborn ability to just sap natural energy into their bodies willy-nilly. Yet they are
sages. They achieved it through training and shit. Neither Oro nor Kabuto were ever trained in gathering natural energy through stillness and all that. Both of them found a way around it to an extent: Oro by modifying the shit out of his body (with Kabuto's help) and Kabuto by doing that even further, adding Juugo's DNA to himself.
Third, I don't know if you know this, but Spiraly Fuck here is on a whole other level of power compared to every other Zetsu
. Oro having a Zetsu body does not magically make him exist on the same playing field as god-tier Mokuton and five-element affinity.