We interrupt this discussion of flower theft for unscheduled archaeological pedantry.
Putting flowers with departed ones goes back at least to Neanderthal times, so, just maybe, there's a good reason for it still being around.
First, "Neanderthal times" could imply that the post-burial recognition of a grave site was a practice of
Homo neanderthalensis, which is not supported by the archaeological record at this time. I'll assume instead that what was meant was instead "post-burial recognition of a grave site was a practice by
Homo sapiens during the time they coexisted with Neanderthals", i.e. from emergence of archaic modern humans ~250,000 years ago to the extinction of Neanderthals ~40,000 years ago.
Second, the practice of leaving flowers or other goods at a grave at the time of burial and leaving them at a grave post-burial are distinct cultural practices. The former predates
Homo sapiens as a species and there is evidence of Neanderthal burials with apparent grave goods. However, the latter has only been found in post-nomadic societies of anatomically modern humans. The earliest grave markers are found 15,000 years ago, which would seem to be the early bound as it'd be difficult to leave anything on an entirely unmarked grave. There is not specific evidence of flowers being left on those early grave markers, though that would be extraordinarily difficult to detect in the archaeological record.
Thank you for your patience with this interruption.