One thing about plurals generally, is that we can sometimes think of them as a single unit.
For example, when we speak of "sports", are we speaking of "the several different activities, each of which is a sport", or are we speaking of "the single activity of engaging in sporting activities"?
The truth of the matter is that you can just as reasonable consider it either way; as a single concept that contains a plurality, or a plurality of concepts.
For the most part this is angels-on-pins stuff, but some plurals get referred to in a close-to-singular way often enough that people tend to think of them more as singular than as plural. And so sales isn't thought of as a collection of activities each of which is a sale, but as the name of an activity that businesses engage in. Sports is thought of as a thing*. Academic subjects are often referred to in the plural (mathematics, economics) but again each thought of as a thing.
And so when people go to use these words as an adjunct, they are thinking of them as singular, and while even always-plural words like trousers and scissors can become singular in trouser press and scissor blade, these words may remain in their plural form in sales manager, sports centre and mathematics textbook.
After all, when we speak and write we generally do not apply the rules rules of grammar in a fully concious way, but automatically. If we're thinking of a plural term as being singular, then we will use that plural term as if it is singular, even if the result does not fit logically with its actual plurality.
*There's some variance of use with sports with sport appearing in some cases as the name for the general activity and some papers having sport sections where some have sports sections, but that at least some usages favour the plural form here suffices to make it fit the pattern.