The source is the English language, do you speak it? :)
Nouns that appear as modifiers in a compound noun are singular, and the need for a singular is so strong that English speakers invent singulars forms of plurale tantum words such as "jean jacket" rather than "jeans jacket".
There are certain cases in which a plural modifier cannot be avoided, mainly because a plurale tantum doesn't singularize. We cannot singularize "goods" in "goods distribution network"; if we drop the "s", we get an adjective which completely changes the meaning.
However, consider the word "supplies" which normally doesn't go to the singular word "supply" denoting one item. Yet in business we have "supply chain", not "supplies chain".
There is a strong urge to eliminate plurals from the non-head position of a compound noun; the urge only loses in cases when a special kind of plural cannot be eliminated.
