I finally got why labor has to be the source of all value, and by value I mean dollar value. I mean what something goes for on the market.
Now, whatever it costs on the market, that depends on a lot of things. But the fact that it costs at all on the market–the fact that it is not free, is not available freely–cannot be for other than the fact than that it could not be at the site where any would-be buyer is without someone's labor. If machines could and did bring, at our command, literally any good–any manufactured object–anywhere at any time, nothing would cost anything. But even though there are machines that can produce some good 100% automatically–say, plastic combs–the fact of their cost to the would-be buyer is still due to the generalized scarcity and unavailability of 100% automatic comb-making machines.
Whether or not such machines exist somewhere else, if the only comb someone could manufacture for themselves would require their actually laboring to produce it, they may begin to be willing to pay someone else money in exchange for a comb.
What's that called? Socially necessary labor time? "the amount of labour time performed by a worker of average skill and productivity, working with tools of the average productive potential, to produce a given commodity."
Capitalism artificially raises socially necessary labor time for those objects by making it impossible for most of society to ever access 100% automatic anything-making machines. If we could share out the access to those machines, the real, specific, on-the-ground socially necessary labor time for the goods they make would drop to zero. I think. The socially necessary labor time to deliver them, however, is still there. But I suppose if we ever have sustainable electricity and Google-driven electric cars, the cost of delivery would fall pretty damned low.