The problem is spiders, which produce the material in very limited quantities, and their cannibalistic proclivities mitigate against high intensity farming.

Well, it looks like the DoD is trying to get silk genetically engineered worms to produce spider silk an alternative:

The U.S. Army is upping its investment in genetically engineered spider silk for body armor. Last year, the service paid almost $100,000 to Kraig Biocraft Laboratories, which makes spider silk that can be produced at scale — with silkworms. On Wednesday, the company announced that the Army will move to the second phase of the contract and will look to Kraig to produce a customized strain of the silk for “high-performance fibers for protective apparel applications.” That is: flexible body armor made from genetically engineered spider silk. The total contract amount would reach $900,000 if parameters are met. Army representatives said that interested in the material purely from a research perspective, for now.

Kraig Biocraft injects spider DNA into silkworm eggs, enabling the worms to produce its custom silk. The researchers describe the process in this 2011 PNAS paper.

Spider silk is much tougher than regular worm silk, and about half as tough as Kevlar. But it’s far more flexible, (3 percent elasticity for kevlar versus nearly 40 percent for spider silk.) The Army believes that the energy absorption of the material could be much higher than kevlar (as determined by multiplying the strength of the fiber by the elongation.)

It’s also much more elastic and flexible than kevlar. But getting enough spider silk to clothe an Army is a tall order. The crawly arachnids don’t produce silk in high volume and when you crowd spiders too close together, they eat each other. The quest to produce spider silk in hosts other than spiders has led researchers to use a variety of other methods such as yeast, e. coli bacteria and mammalian cells. 

There is also the fact that the techniques for handling silkworms, and harvesting the silk, have been known for thousands of years.

It is also far less alarming than the prospect of an escape of motherf%$#ing mutant spiders.