The earth is a dynamic place [...] with multiple different processes of deposition and erosion under way at all times. You can make guesses based on style and weathering, but fragments of worked stone that have been in the open for an unknown period can't be dated by their archaeological context, because there is none. Carbon-dating organic materials in the sediment in which they were found won't work, either, because they were never entombed and preserved in sediment. And in fact no other objective and widely accepted method of dating can tell us how old they are. For these reasons archaeologists have to discount artifacts found on the surface when coming to any conclusions about the age of a site, even though the artifacts themselves may obviously be ancient.