Why has nobody ever farmed spiders for silk?

They eat each other. Spiders are territorial and cannibalistic, so they cannot be kept at density the way silkworms can - put a thousand in a tray and you get rather fewer, larger spiders. Silkworms are docile, sessile, and have been bred for five thousand years into an animal that cannot even fly away.

The silk is worth wanting. Dragline spider silk is tougher than steel by weight and tougher than Kevlar, absorbing more energy before breaking than almost any material, natural or made. The one large-scale demonstration is a piece of cloth made in Madagascar in the 2000s from golden orb-weavers: it took over a million spiders, collected each morning, milked and released, and eight years of work to produce a single embroidered shawl.

So the effort has gone into making it without spiders. Genes for silk proteins have been put into bacteria, yeast, goats - producing it in milk - and into silkworms themselves, which is the approach that has come closest, since a silkworm already has the machinery to spin a fibre. The hard part is not the protein but the spinning: a spider extrudes it through a duct that changes acidity and pulls the molecules into alignment, and reproducing that has proved much harder than reproducing the chemistry.