It's irrelevant whether professors work outside their universities though most do .Often the applications for a persons scientific discoveries are years, decades or centuries away but some people are more interested in intellectual stimulation than fancy cars. Most professors have upper -middle class incomes and most are married to a professional with a sustantial income.
Richard Feynman is considered by many to be the smartest person of the tentieth century . He spent his life as a Caltech professor. Several of my law school classmates gave up million dollar a year law firm partnerships to be on the Fed Ct of Appeals at drastically lower pay because they find it more interesting.