This article by Frank Pestano from SunStar.com may greatly explain this. They too have "other" works to look and take after to earn a decent living.
Mikhael Botvinnik was a three-time world champion who was an electrical engineer. His famous pupils include world champions Anatoly Karpov, Garry Kasparov and Vladimir Kramnik.
Max Euwe was not only a world champion but was also a past president of Fide. He was a professor and had a doctorate in mathematics.
Paul Morphy was considered by Bobby Fischer to have been the greatest player of all time and an unofficial world champion. He was a lawyer by profession and was known to have memorized the complete Louisiana book of code and laws.
Miguel Najdorf was a Polish-born Argentine chess grandmaster of Jewish origin, famous for his Najdorf Variation. He was an insurance underwriter and once played communist revolutionary Che Guevara to a draw.
John Nunn is one of England’s strongest players and once belonged to the world’s top 10. He is also a college instructor in mathematics.
Reuben Fine was one of the strongest chess players in the world from the mid- 1930s through the 1940s and is a psychoanalyst.
The full article from SunStar, 'Occupations of Famous Chess Players' is re-posted at Pestano's chess blog, Chessmoso