Because we consider one (pair of) jeans / one (pair of) trousers as a singular in German.
Both "Wo ist meine Jeans" and "Wo sind meine Jeans" should be allowed for one single pair of jeans (cf. https://www.duden.de/rechtschreibung/Jeans_Hose). The plural is mostly used by older people, in my experience.
I don't understand why here Jeans takes singular in German. In English it takes plural and Jeans, according to Leo, is a plural noun even in German.
But it take "meine" for plural !!
Nope, “meine Jeans” is feminine singular. I know, weird, right?
What's wrong with Wo sind meine Jeans
That would mean you're looking for multiple pairs of jeans.
No, its feminine