When Warren Buffett was asked why he's not buying Amazon shares, the billionaire investor had a simple answer.

"Stupidity," Buffett said during an interview Monday on CNBC's "Squawk Box."

The billionaire investor also heaped additional praise on Amazon.com founder Jeff Bezos after commending him over the weekend at Berkshire Hathaway's annual shareholders meeting.

"I was impressed with Jeff early. I never expected he could pull off what he did ... on the scale that it happened," Buffett said Monday. "At the same time he's shaking up the whole retail world, he's also shaking up the IT world simultaneously."

"These are powerful, powerful ideas with big potential, and he's executed," Buffett said.

On Saturday at Berkshire's meeting, the "Oracle of Omaha" said he missed an opportunity on Amazon and made a mistake by not buying Google shares, now traded under parent company Alphabet.