There is a general tendency in languages that when a word that inflects is incorporated into a word or a phrase as a modifier, it loses its inflection.
Since we don't have many inflections left in English, this is not as obvious as in some other languages; but it is generally the case that when a noun is used as a modifier in English it does not take plural inflection irrespective of the sense:
cow house, dog kennel, car park(ing), tree surgeon, window cleaner, bookseller, flea circus, language lessons, container ship, crop spraying, child poverty ... the list is endless.
There are exceptions of course: "drinks cabinet" is an example. But in the overwhelming majority of cases (including all measurements used attributively) the qualifying noun is in the singular.