All new lenses are wrapped in a thin plastic bag. They are then put in styrofoam pieces that help to seal the plastic bag. The styrofoam, of course, helps to protect the lens during shipment in the retail box or from accidental drops. Essentially the lens are sealed from moisture from the outside environment. Moisture or humidity encourages mildew or fungus to grow on your lens.
Your home environment is different. Your lens are not wrapped in anything when you are using it or during times when you leave it in your camera bag or any storage enclosure. If you do not use your camera kit for several months and kept in the dark there is a risk of fungus growth. If you use your camera frequently it is less likely you will face this problem.