From what I heard okobo are worn by maiko because back in the old days most maiko would still be children (much younger than the regular minimum age of 15) so okobo would make them look taller. Since geiko are supposed to be mature women they don’t need okobo.
Another source says that okobo are supposed to make the maiko look a bit clumsy wearing them - like the clumsy walk of a child but I’m not sure about that actually.