But I don't want spider silk in my goat milk!
Neither do the baby goats! The purpose is to harvest the spider silk for its strength in certain special purpose fabrics and applications.
Genes from the spider were transplanted into the goats, but I don't think they bothered to make it possible for the babies to digest it. It has to be strained out of the milk.
One goat can produce millions of times more spider silk than an army of spiders. Not to mention there's a shortage of those tiny little stools they use when milking the spiders.