Many possibilities:
- Zetsu is Shinju Reincarnation
- Zetsu was created of Gedo Mazo, this way, Kaguya's will was passed down to him
- Zetsu was there even before Madara VOTE battle
- Madara's will had Kaguya's will inside of it, this way, Zetsu felt like Madara was following the wrong path and had to take the lead
- Madara's will is Zetsu's will, there can't be two Madara in a single world
- Zetsu is the land, he's tired of humanity
- Once Madara reached Rikudou's chakra, he did reach Kaguya's chakra, too. So it left an open door for Kaguya
There are so many options, seriously ol