Why can't the word stewardess be used in academic works in western countries?

Because stewardess is a general term for a position:

A flight attendant's career can be divided into four stages: general cabin attendant, cabin attendant, regional flight attendant and chief flight attendant.

Academic works are very rigorous, and the word flight attendant that is not clear can't be used.

Extension:

The earliest flight attendants appeared at 1930. A 25-year-old nurse named ellen church was hired by United Airlines to take care of the passengers on the plane. For a long time, flight attendants were mainly women, but more and more men joined the ranks of flight attendants. Because flight attendants attach great importance to women's appearance, beauty is also one of the conditions for becoming a flight attendant.

Flight attendants need to receive training provided by airlines before joining the job, and they can only formally take up their posts after passing the qualification. The training content includes service, etiquette, make-up, aircraft safety and first aid. To make sure you know how to deal with an accident. In addition, all prospective flight attendants must obtain permission from ICAO to become flight attendants. In order to communicate with passengers from all over the world, flight attendants usually need to know English and one or more other languages.