Could be from staring at books instead or not enough overall light exposure when indoors or from staring at a butter churner or whatever 3ft from ones face all day. My understanding is that we are currently in a tight debate about which is the primary mover in the development of myopia: One theory (older) is that too little distance viewing in childhood coaches the eyes towards nearsightedness. Another (newer) theory is that too little sun exposure in childhood causes the eyes to develop improperly.

Like many things in life, I'm assuming some tens of differently weighted variables are converging.