To succeed as a lichen, you don’t need a rock. Sure, lots of lichens are found on rocks, but it’s not a hard-and-fast requirement. What you really need is a place no-one else wants much, where you can grow your 1 or 2 mm per year undisturbed.
This one has been left alone on this forest floor long enough to be the size of a luncheon plate. Nothing wrong with being the slowest “tree” in the forest!