Why are unmarried men called bachelors?

In 1930s, there was a film which reflected the life of the poor in shanty towns and contained anti-Japanese content, starring famous movie actors Langen Han and Cilong Wang. The interlude in the film was sung by Langen Han and Cilong Wang. The lyrics are "If you are single, you say that your life is really hard. You lived in vain for 35 years, and your clothes are torn. Well, if you are single, your clothes are torn, and no one can make up for it. If you feed, there is water but no rice in the pot. Being fed is poor, and I miss you every day.