Why doesn't the fat in pemmican mold?

Mold spores are everywhere, but they need certain conditions in order to grow. Food preservation and storage steps protect against spoilage. Mold growth is one (of many) potentials for spoilage (although in some instances, cheese for example, it is desirable). Fat doesn't necessarily encourage mold growth any more than other products. The conditions that mold spores need to grow are (a) water, (b) food in the form of dead or decaying matter, (c) air, and (d) temperature (70F or 20C is ideal). In the case of pemmican, the water activity of the product is very low, as it is made mostly from dried meat and fat. During the time that it was used and popular, it was probably protected from the air as much as was reasonable, and it was used and stored (mostly) in cool temperatures. None of that guarantees that it will not, or never did mold. I can imagine an early Native American, or North American explorer reaching into a leather saddle bag after the spring rains and pulling out some moldy pemmican. In that scenario, the conditions could be perfect for mold growth. So, to directly answer your question, it's not the fat or the pemmican. Mold can grow on anything that supports its growth if the conditions are right.