Seasteads cost money, and if you want to succeed as a Seastead you have to find ways to attract people to move there. If I was a billionaire I wouldn't want to move to a seastead, but if I was a member of the bottom billion, most of whom want to leave their dysfunctional governments, I might want to move to a seastead.
People can leave seasteads, or people can choose them, and people can create new seasteads if they want. This fluidity will engage an evolutionary market process that'll allow a diversity of societies to emerge that will in principle be superior, simply because people chose them. Governments on land don't allow this fluid dynamic of choice.