> Why not? Sculptors sometimes make dozens of sculptures per year, by themselves.
Sculptors today are stationary and they depend on money (a mark of a civilization) and tools (same) and supermarkets (same) and demand for their work (economic surplus; same). Leonardo da Vinci, if he was a part of hunter-gatherer society (which is mobile), would neither be able to paint nor invent, for he would be busy with gathering (or growing, or hunting) his own food most of his time; he would also not have access to paper or canvas or colors or brushes.
> I see no reason whatsoever to assume that those sculptures weren't created by part-time artists.
As mentioned, hunter-gatherers are mobile at least part-time, because they go from where some food grows today, to where some other food roams tomorrow, to where yet other food swims the day after.
> Social structure exists even in apes
And bees, and ants; but not the kind that allows a big part of a society (capable adults) to engage in activities that are not directly related to gathering food, growing children, or fighting, but creating temples and art. A group of hunter-gatherers can provide for themselves, their children, and maybe their elders or sick (as long as these are mobile). They can even accommodate a leader or a shaman here and there; but not an apparently big group of able adults that do not hunt and do not gather and do not fight and do not move but rather sit in one place and build a temple and statues.
> there is nothing substantially new in this archeological find
Gobekli Tepe have completely changed our view of the development of the human societies, as we have previously believed the first civilization (Sumerians) to be as recent as 5,000-4,000 BCE (or 6-7 kya). This hints at a civilization twice as old as previously believed.