In the light of geographical phenomena, explain why India (including Pakistan and Bangladesh) and China have accounted for close to 60% of human population at various points in human history.

Though China, India, Pakistan and Bangladesh occupy just more than one tenth of land surface but have been home to 60% of the human populations at various points in human history. Nowhere in the world can one see this type of population concentration.
The main geographical factor for this is the presence of multiple river systems in these countries. India (including Pakistan and Bangladesh) and China had multiple river systems that were both massive as well as hospitable that lead to huge food production. Instead of depending on a single river system (like Egypt), India and China each had more than 3 major river systems giving a sort of backup. Thus, when Indus valley civilization faltered, the civilization quickly picked up the slack near Ganges. Presence of more plain areas, that leads  to high presence of populations due to fertility of soils.