Why are truffles among the most expensive food items on Earth?
Because truffles have to be not picked but found underground, usually with the help of a specially trained mushroom-sniffing dog. Species of truffle (in the Tuber genus) are ectomycorrhizal, requiring a symbiotic relationship with roots of specific trees to live. The truffles, sprouting underground attached to the roots, get quite easy access to the nourishing sugars that are created by the tree during photosynthesis. The tree gets the benefit of increased root surface area with which water and nutrients can be better absorbed.
Add to that lag times of up to 20 years before truffles begin to sprout.