This is because humanists often rely on texts that can be extremely old. The field accepts them as authorities. Humanities research can stay viable for decades. Humanists commonly research a primary source such as a sculpture, painting, poem, or bibliography of a person. Primary sources are also books that are used in research as opposed to a journal, which is considered a secondary source.
Secondary sources are ones that pull from other resources for their content.
While secondary sources are utilized in humanities they are not as prominent as they are in scientific research. Textual primary sources typically provide historical content that is the foundation in humanities research. Scientific research is dependent on timely information. If it is out of date it is useless for the science researcher. Text is very important to the humanities not only in research but also in the physical form.