The nasals are more freely combinable: a nasal may either precede or follow a mute of either kind, or the sonant spirant h; it may also follow a surd aspirant (sibilant); no nasal, however, ever precedes a sibilant in the interior of a word (it is changed instead to anusvāra); and in external combination their concurrence is usually avoided by insertion of a surd mute. (From Sanskrit Grammar by William Dwight Whitney
Why is the anusvaara never written as the homorganic nasal consonant before a sibilant (श, ष, and स)?
Is the it not pronounced the way I think ( ञ्श, ण्ष, and न्स respectively) or is there another reason I am unaware of?
कंस is never written as कन्स
अंश is never written as अञ्श
and दंष्टरा is never written as दण्ष्टरा (but I have no experience with this term; I just trusted Whitney here)