The point of academic talk is to try to persuade your audience to agree with you about your research. This means that you need to raise a structure of argument in their minds, in less than an hour, using just your voice, your slides, and your body-language. Your audience, for its part, has no tools available to it but its ears, eyes, and mind. (Their phones do not, in this respect, help.)
This is a crazy way of trying to convey the intricacies of a complex argument. Without external aids like writing and reading, the mind of the East African Plains Ape has little ability to grasp, and more importantly to remember, new information. (The great psychologist George Miller estimated the number of pieces of information we can hold in short-term memory as "the magical number seven, plus or minus two", but this may if anything be an over-estimate.) Keeping in mind all the details of an academic argument would certainly exceed that slight capacity*. When you over-load your audience, they get confused and cranky, and they will either tune you out or avenge themselves on the obvious source of their discomfort, namely you.
Cosma Shalizi at Three-Toed Sloth