That is all true, but the idea of seasteading is to explore the options for high-quality governance outside of those available to any presently existing countries, which are severely curtailed by their domestic historical and political constraints, as well as international entanglements. Of course, the proposition that there are such options is somewhat controversial, though I find it pretty evident that there should be. (I am however much more skeptical about the possibility of building them from scratch in a planned way, even if the problems of ensuring sovereignty are solved somehow.)

Also, you forgot Liechtenstein on your list. It is perhaps the best example of a highly successful small country that prospers by offering high-quality governance that attracts business. (Recently I was amazed to find out that rather than being just a finance hub and tax haven, more than 40% of their labor force is in manufacturing!)