Strands of spider silk are not a simple uniform material and are far more complex than plastic fishing line. Plastic fishing line is a solid, uniform entity. A strand of spider silk is an organized structure built from small units.
When we observe a strand of spider silk, we only see the outer skin. The outer skin is composed of glycoproteins. Inside this visible layer is an inner skin that is rich in lipids.
The skins surround and contain bundles of independent fibers (think or a box of spaghetti: spaghetti=fibers; box=skin). An individual fiber in a bundle is not a simple uniform material. Fibers are ordered assemblies of silk proteins. At least 2 of the silk proteins have been characterized and they have similarities and useful differences. The quality of the fibers and thus the quality of the silk depends on the types of silk protein, their ratio and their arrangement. Protein mix and match results in strands with a potentially large diversity of properties.
Arthropods assemble silk on a nanoscale with cells being the unit of protein and lipid production. The silk fibers self assemble in a nanoscale environment and are joined and bundled at a larger scale. We can produce buckets of spider proteins at a very large scale. However, the interactions of the proteins and their self assembly are different at the much larger scale of a test tube. To manufacture synthetic spider silks will require developing appropriate tools at the nano-scale.