Why does a course in World History not often rely on written sources as evidence? What sources do they use in lieu of written sources?

Based on my experience taking the class, they do use written sources as evidence in which vary in primary and secondary sources. It's possible that they do not rely on written sources because of bias, but the students usually have to take that into account when making connections in essays, depending on whether the source is from a trustworthy person or not, such as a king or a philosopher.