i.e. there is no "elsewhere" for them to move
Which may have been the case, I remember a line from the movie Soylent Green, that the farm areas were protected like fortresses. People stayed in the cities, because they literally couldn't get out. They weren't permitted out.
Consider, currently urban and suburban areas account for three percent of the land area on Earth, agricultural crop land is around eleven percent of Earth's surface, and pasture range land is another approximately twenty-five percent. Now multiple the Human population by twenty
(or a hundred
) times. Even if you make us all vegetarians, and increase crop yields, you can't have the bulk of the population spreading out into the agricultural lands. The same would apply to living on top of the majority of the water surface, you're pulling food out of the water, you have to leave it alone. The food has to come from somewhere.
Most of the Human species would likely still be in the same three percent of the land area.