This only applies to some of the translations, but there's actually a very simple reason: chemist does not rhyme with Trismegist, while alchemist does.
The word chemist has the stress on the first syllable, and would require a double (or feminine) rhyme like menaced—which is still only a very good near-rhyme (I actually can't think of any perfect rhymes ...
Baudelaire's Les Fleurs du mal contained two poems with almost identical titles:
"Femmes Damnées (Delphine et Hippolyte)"
When Baudelaire and his publisher were prosecuted for obscenity and blasphemy in 1857 (by the same lawyer who had prosecuted Flaubert),
six poems were to be removed from Les Fleurs du mal, including "Femmes Damnées (...