Because footprints are made in soft ground, like mud, clay, etc. The edges are soft and rounded. Anything re-exposed to to the surface and weathering conditions again, only wears down and "softens" further. The edges on your rock are jagged, which is all thats needed to see its not a cast fossil footprint.
I looks like it might be a print, but I am leaning towards it not being so. Your host rock is quartz, and I do not believe that it would be possible for that to occur. Your host rock should be more of a hardened stream deposit.
It doesn't look like quarts to me, doesn't have any crystalline structure to it. It may be the photo isn't doing justice. Looks like it has similarities to bed rock material of some sort even though it may or may not be that.
I should note then, that the place this was found was originally an ancient swamp millions of years ago. Not sure if that solidifies the thinking behind what this could be, but it seems important nonetheless to note.