I'm confused now. Why is the נ in the plural and not the singular?
Phonological change. The original Semitic root is אנש. When the stressed feminine suffix -āh
(originally *-atu) is added, it causes the medial vowel to drop out so there's nothing between /š/ and the preceding /n/, which then assimilates to it, i.e. *`anaš-atu > *`anšāh (lenition, syncope, and apocope) > *`inšāh (Barth's law) > `i:ššāh (assimilation and pretonic lengthening). The plural form must've had a different stress pattern which prevented this from happening. Instead, it was the short initial syllable which got dropped.
Oh, okay, so the internal ablaut is the exception, and not the rule?
The usual way in which this manifests is reduction of the initial syllable. For instance, /da'var/ "word" to /dva'rim/ "words". There are some more complex examples like /'melex/ "king" > /məla'khim/, but (a) they're not especially common and (b) if this is enough to scare you away from Hebrew then, whatever you do, don't look at the verbs!