They don’t need to cash out their stock. Banks loan them whatever they need with their stock as collateral and their interest rate is near zero. That’s how they do it.

And because they aren’t realizing their capital gains, they don’t owe taxes on their accumulated stock wealth.

https://www.propublica.org/article/the-secret-irs-files-trove-of-never-before-seen-records-reveal-how-the-wealthiest-avoid-income-tax