Humans were not mass manufacturing things back then, and there were orders of magnitude less people.
On top of that, we don't even have exact knowledge of where to look. Most places we do discover have plenty of signs and evidence, they are just buried under 10ft of dirt and thus essentially undiscoverable.
> Humans were not mass manufacturing things back then
Ah, but how do you know that? You weren't there. ;)
Sometimes the absence of evidence really is evidence to the contrary, but other times it isn't. It all depends on whether or not it's reasonable to expect evidence to be found given the amount of looking we've done and the nature of the evidence we're looking for. If we don't find a ton of glass coke bottles in the soil around the world, after digging around in innumerable construction sites on almost every corner of the globe, that's strong evidence that nobody was mass manufacturing coke bottles 50k years ago.
Contrast that with the lack of evidence for ancient wooden sailing vessels. We don't have any evidence of wooden ships 50k years ago. But supposing there were a shipbuilding culture back then, would we really expect to find evidence for it? All those wood artifacts would be LONG gone, even the oldest bog wood ever found is less than 10k years old. In this case, the absence of evidence is weak evidence to the contrary at best.
Old things aren't necessarily that far under our feet. I've excavated 4,000 year old sites under a couple inches of dirt, and found 10,000+ year old artifacts on the surface.
On top of that, we don't even have exact knowledge of where to look. Most places we do discover have plenty of signs and evidence, they are just buried under 10ft of dirt and thus essentially undiscoverable.