Well all the other children and past children have drawn it yellow, so it is a part of the culture of children's drawings. The culture really does matter, for e

Well all the other children and past children have drawn it yellow, so it is a part of the culture of children's drawings. The culture really does matter, for example young children in different cultures draw human figures quite differently. As someone else noted in these comments, you can't draw white on white paper well, and yellow is the closest thing really in a box of crayons, so that probably has a large effect as well.