In an interview with CNBC on Monday, Buffett said that his company has “bought more Apple than anything else” in the last year. As of this writing, Apple is Berkshire Hathaway’s second-largest stock holding at $28.2 billion. Wells Fargo is Berkshire’s biggest holding at $29.3 billion. Bank of America, Coca-Cola, and American Express round out the top five.

Buffett told CNBC that he’s attracted to Apple on a number of fronts, not the least of which is its “extraordinary consumer franchise.” He added that Apple’s ability to keep customers coming back is also appealing to him as an investor.

“You are very, very, very locked in, at least psychologically and mentally, to the product you are using,” he said. “[IPhone] is a very sticky product.”

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That “stickiness” has been an Apple hallmark for years. The company has what’s called a “closed ecosystem” that encourages (or some critics might say, forces) users to employ its apps and services. An iPhone, for instance, comes only with Apple’s App Store, Safari browser, and Apple Music streaming service out of the box. While users can download browser and music alternatives, many rely on the company’s services. That reliance makes moving to an alternative, like Android, even more difficult, since many of Apple’s services are only available on its products.

That strategy has helped Apple generate billions of dollars each quarter over the last several years and aided the company in growing its massive cash hoard. It’s also made Buffett and other investors quite pleased with their investment.

Still, there’s one user that Apple has yet to sign on: Buffett himself. The famed investor still uses a flip phone, he told CNBC, and said that he’ll likely be the last person to ever buy an iPhone.

The reason? “I’m out of touch.”