Why doesn't “astronomy” end with an “s”?

This is from etymonline :

in the names of sciences or disciplines (acoustics, aerobics, economics, etc.), a 16c. revival of the classical custom of using the neuter plural of adjectives with Greek -ikos "pertaining to" (see -ic) to mean "matters relevant to" and also as the titles of treatises about them.

Subject matters that acquired their English names before c. 1500, however, tend to be singular in form (arithmetic, logic, magic, music, rhetoric). The grammatical number of words in -ics (mathematics is/mathematics are) is a confused question.

Greek words give rise to the '-ology' endings from λογος, logos - 'word'; to the '-nomy' endings from νομια, nomia - 'rule' or 'law'; and to the '-ics' endings from ικος, ikos - 'pertaining to'.

It is from the Latin and Greek suffix -ia which in English has become -y, probably for assonance with the original French astronimie:

Astro -nomy

a combining form of Greek origin meaning “distribution,” “arrangement,” “management,”:

• astronomy; economy; taxonomy.

From Greek -nomia law. nomo-, -y

(Random House Dictionary)

Astronomy:

c. 1200, "astronomy, astrology, scientific or occult study of heavenly bodies," from Old French astrenomie "astronomy, astrology," from Latin astronomia, from Greek astronomia.

-y

suffix indicating state, condition, or quality; also activity or the result of it (as in victory, history, etc.), via Anglo-French and Old French é, from Latin -ia, Greek -ia, from PIE *-a-, suffix forming abstract or collective nouns. It is etymologically identical with -ia and the second element in -cy, -ery, -logy, etc.

(Etymonline)