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Probably because you have no idea what you're looking at...
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Although the photo is lousy, anyone who has experience in successfully replicating stone tools can readily see evidence of planned and controlled bilateral, multiple flake removals on some of the specimens. That simply does not occur by any natural ("geofact") process.
There are three specimens in the photo that show repeated, adjoining flake removals -- positioned to create a sharp cutting edge on one lateral side. All flake removals were made by "hard-hammer percussion" (hitting near an edge with another rock ["hammerstone"] -- specifically to remove flakes from the opposite face of the piece).
The specimen at right, near the top of the 10-cm scale stick, has almost a dozen such carefully-placed "flake scars" along its right edge alone. Given the grainy toughness of that stone material, I'm not sure I could do much better with the same piece of rock.
There is zero question that the mind-eye-hand feedback loop of a human being produced some of the pictured specimens.
As to their actual age, deponent sayeth naught... '-)
TXnMA 
Texas Archæological Steward, lithic technologist, flintknapper...