Countries which the land area is mainly made of mountains like Greece, Nepal, and some european countries have relatively small and stable and even declining population so I get that if the vast majority of Earth land surface was made of mountains the global population would be much smaller due to litits in agriculture. It is much harder to make crops grow in a irregular terrain than it is in the plains. Transportation would be also harder because it is harder to build roads and rails in mountains.

Countries with land areas mainly consisting of mountains like Nepal, Greece, and some other European countries have relatively small and stable and even declining populations. So I get that if the vast majority of the Earth's land surface was made of mountains the global population would be much smaller due to limits in agriculture. It is much harder to make crops grow in an irregular terrain than it is on the plains. Transportation would be also harder because it is harder to build roads and rails in mountains.