1) The seasteading.org folks aren't rich, though they can get resources from rich people who like their program.
2) They have a well thought-out program for making their own land.
3) The land they have is very small, and it's perched on top of tall towers, therefore easy to defend. Private security and agreements with local governments should suffice for their needs.
4) The seasteading folks plan to use flags of convenience, or plan to just not be noticed. The platforms will most likely be mobile, so if they don't like their neighbors, they'll just move.
5) Since each unit is relatively small, they can use the same legal framework that captains aboard oceangoing vessels use.
[1] The seasteading folks seem to be about Libertarian ideals of freedom. I understand that there are indeed some folks willing to die for those ideals. I am not saying if this this is a good idea or not.
Disclosure: I am not a member of seasteading.org. I just think they have neat engineering ideas.