Why can't humans build cities on the sea in the future?

Mars is the United States teasing you, teasing the world, and the United States is fooling. Without the earth, it is impossible for human beings to survive. It is impossible for human beings to survive without land, and there is no existence of building cities on the sea.

There is a movie in America called Future Water World. After reading it for a while, I felt that it violated the objective law, so I didn't look down. .