The warehouse club has always paid higher-than-average wages as its leadership seems to understand the value of holding on to workers. That's especially evident when you consider the career trajectory of its new chief executive, Ron Vachris.

"Ron is a Costco veteran, with over 40 years of service to the company, starting as a forklift driver, and subsequently serving in every major role related to Costco's business operations and merchandising activities," the company said in a news release.

That's not a common path to the top job at any company, but Costco has generally done a good job of retaining staff. The warehouse club has an "A" rating on Comparably's tracking of companies' retention of workers.

"Costco is in the top 10% of similar-sized companies in its ability to retain quality employees. 58% of employees would not leave Costco if they were offered a job for more money while 68% are excited to go to work each day," Comparably said. 

The chain also ranked No. 1 compared with its competitors, which include Walmart, Amazon, Best Buy and Target. 

Costco, however, now faces a challenge that could affect how it retains its employees and operates its business.