So I guess you are suggesting that aliens should have drilled oil wells, built oil refineries, built polymerization of ethene plants, and plastic manufacturing plants instead of just cutting stones from the ground. Or digging iron ore mines, digging coal mines, combining the coal (coke) and ore in a blast furnace that they also would have to have built, produce the pig iron, and then manufacturing steel from the iron through an oxygen furnace, instead of just cutting stones from the ground.
Seems to me that it's a lot easier and quicker to just cut stones from the ground, instead of investing all that time and energy in building mining and processing plants. Seems to me too, that they did contribute engineering expertise in helping to build all those megalithic buildings.
Let's backtrack a bit. I'm talking about "ancient aliens" hypotheses, of which I'd suggest there's a weak version and a strong version (weak or strong in boldness, not evidence). The "weak" hypothesis is that sentient beings from another planet traveled through space, visited Earth, and sufficiently impressed ancient Earthlings that they were remembered as gods or some such and inspired mythologies. The "strong" hypothesis is that these aliens also left a mark on the societies they visited, specifically by building or assisting in the building of monumental structures, like Stonehenge or the Pyramids. They did so by some combination of assisting in the design of the monuments, aligning them with the poles or astronomical phenomena etc., or in the actual construction, moving stones around that are allegedly too big for low-tech human societies to move.
Let's not worry about the "weak" hypothesis, this thread is about technology. To be able to make the trip across space, the aliens would need to have far more advanced science and tech than we now have. We have no idea what all they might have been capable of besides space travel. Maybe not much, and they were the equivalent of Mercury astronauts in just a tin can spaceship with huge hyperdrives or something pushing them; maybe a whole lot, and they were the equivalent of a whole aircraft carrier pulling up to an island with stone-age natives inhabiting it, bringing myriad devices and materials with them.
These ancient aliens CAN'T have had a Star-Trek style "prime directive" not to interfere, because the whole premise of the hypothesis is that they DID give their technical aid to the primitive Earthlings.
So if, on the one hand, they contributed to the design of the monuments, why don't the monuments show the signs of design with strong physics knowledge? They aren't even up to medieval levels in their engineering skills.
On the other hand, if they didn't contribute to the design, then the monuments were already designed by those human societies for human purposes. But for some reason the aliens decided to help the humans build something they couldn't have done on their own, lending them the use of their spaceships to airlift big stones into place. But then why were the Earthlings designing monuments they knew they wouldn't be able to build?
As far as the issues of plastics and metals that you keep returning to; that isn't really the nub of the argument. But it's not irrelevant, either. If we're envisioning aliens in a tin-can space module, they wouldn't have much materiel to contribute to a construction project; all they could give was design advice, in which case, see two paragraphs up, and we'd have to acknowledge the Earthlings could move the stones without alien help.
But if we're talking about nuclear-aircraft-carrier visitors, they would have been stocked with spare parts and tools and machinery and so on. Maybe it WOULD have been worthwhile to them, and within their capabilities, to set up a manufacturing chain for plastic or glass or steel. We're talking beings with capabilities far beyond what we can manage, maybe that wouldn't have seemed too much to do for them.
But even without that, any device it would have been in their interest to build for humans, surely they'd think could be done more efficiently than with massive stones. They'd at least jerry-rig something out of materials they had with them, and it would be odd that they left huge stone monuments behind but none of that other stuff. Think of the enormous amount of junk that cruise ships generate. The modern San ("Bushmen") in Africa, notwithstanding the romantic primitivism of "The Gods Must be Crazy," actually DO have Coke bottles and cans and wristwatches and such, things they can't make themselves but picked up through contact with peoples who do have them. Where are the alien Coke bottles in Egyptian sites?
He got ALL the girls, plainly. It is written.
