He doesn't tend to show up in the lists because he pays himself relatively little (compared to other billionaires). Between 2014-18 he had income of $125m, and paid tax of $23.7m[1]. That's still lower than the average American absolute tax rate, but way higher than most other billionaires. So he's a bit less of a sensational headline than his contemporaries.

His wealth growth of $24.3bn is untaxed through those years, he presumably left those assets in Berkshire Hathaway where he'd personally not generate a tax bill for them. Eventually Buffet, or more likely his inheritors, will sell the Berkshire Hathaway assets, and should pay a monumental tax bill on them.

[1] https://www.propublica.org/article/the-secret-irs-files-trov...