Now, a new study shows a pretty compelling reason why.

Writing in the California Sun, editor Mike McPhate says the average annual salary of an In-N-Out manager is now $160,000. 

That's a lot of money, especially considering that it's a path to a six-figure paycheck that's open to people who don't have a high school diploma--never mind a college degree. 

Compare it to the average adult in the United States who does not have a high school diploma, and earns just over $20,000. 

(If you don't want to work in fast-food, maybe check out this recent ranking of the 50 best jobs in America.)

These In-N-Out managers make more than the average salaries of lawyers, software engineers, and architects in California (about $115,000 for the first two professions; $112,000 for the architects).

"In-N-Out is just eons above everybody else. On wages and benefits, they really are the best large chain," Saru Jayaraman, of the Food Labor Research Center at the University of California, Berkeley, told the Sun.

With only 329 restaurants, In-N-Out is small. 

Compare it to about 37,000 McDonald's locations. Even Five Guys has 1,500. 

It's also focused squarely in the Southwest and the Pacific. This despite signs that people on the East Coast (especially former California residents like yours truly) would welcome its arrival. A fake "In-N-Out Coming Soon" sign was once called the "cruelest April Fool's joke ever" when it was posted in New York.

But there's reason to believe that In-N-Out's size, structure, and regional focus are some of the reasons why it's able to offer higher-than-average salaries in the first place. 

It's a private company.

Big public companies that have to answer to shareholders will always have an incentive to pay the lowest salaries they can get away with--assuming acceptable levels of turnover, product quality, customer satisfaction and the like.

In-N-Out however, isn't public; it's still controlled by Lynsi Snyder, the 34-year-old granddaughter of its founders, Harry and Esther Snyder. If she and her team think paying higher salaries makes sense (clearly, they do), they're free to do so.

It's (mostly) in California.

Both wages and the cost of living are higher in California than most other places in the country. In-N-Out workers get a starting salary of at least $13, including benefits. That starting pay can't help but affect the salaries of people who hold positions of greater responsibility.

The company's entry-level workers will be getting pay raises soon, too: the state's minimum wage is $11 an hour currently, but it's set to go to $15 by 2022.

It pays off.

Here's the bottom line: In-N-Out pays more because it works for them. A managerial job at the burger chain is hard work, requiring long hours.

Nevertheless, as McPhate points out, 90 percent of employees on the career site said they'd recommend the company to a friend.

"Our research shows that companies that take the high road make a profit not in spite of paying their workers better but because they pay their workers better," Jayaraman told the Sun.