There is no significant unclaimed or uncontested land left on Earth on which to establish a nation, whereas artificial islands in the form of ships outside the national waters of any state can quickly be recognized as a nation without upsetting the international diplomacy.
Well-built self-sustainable ships in waters without major tsunamis are also safe from almost all natural disasters, as well as most of the risks of societal collapse. With Forever Fleet meant to help wherever needed due to climate crisis, in many cases due to rising seas, the ships will be needed for this anyway, making them an efficient combined solution.
The ships' size limits ensure that maximum populations in direct contact with each other are limited, reducing the risk of internal conflicts.
Forever Fleet will have many other kinds of vehicles for both water, airborne and ground use, as dictated by the situation. Perhaps most notably, Hybrid Air Vehicles will be used for moving containers and people wherever and whenever needed.