Actually they DO grow fungus. The humidity in some central aircon system is actually not as low as we think. My office is >55%. I personally have seen lenses stored in funan growing fungus, but I am not talking about camera shops though, just camera enthuist.
Brand new lens sitting in airconed shops can grow fungus. This is especially true for lens that are not very popular and see very slow stock movement. For instance, when I searching for the sigma 150mm macro, out of the 3 copies that I looked at, 2 were showing signs of early fungus growth. It is only obvious when bright led light is used to view.
Because fungus spores have to enter the lens to start thriving. In the factories, there is a clean environment whereby the amount of dust and microorganisms in the air are monitored and actively cleared. (I hope so, or only the sensor and processors are manufactured in ultra clean environment?)
Unless you open the package, test the lens in the shop and put it back in, no fungus spores will be introduced to the lens. it will sit in the packaging with no foreign matter intoduced to it.