This is what I would bet on. If you spent decades of your life knapping various "easy" and ideal stones for tools and getting quite skilled at it, it doesn't seem like much of a stretch for them to try knapping a "difficult" stone just because it looks cooler and you can show it off.
Not just people. Many bird species collect 'novel' items when making nests to attract mates. In birds we think this may be a means for a mate to measure the fitness of the male, that is they have the resources, intelligence, and ability to find these novel items, then it's likely they'll be a good father to their offspring.
It doesn't seem far fetched that this started from a similar place in humans, but has been wildly adapted in the time since as our abilities grew and the complexity of our society increased. Very hard to prove any thing like this though.