No one is carrying that amount of cash. The rule is from the Jurassic era more or less, and immigrations officers know that bank accounts have since become common. It’s only required if appearance is like a bum or you come from a poor country.
sure could you imagine if there was a major announcement in the media that all people entering Thailand need to carry 20,000 baht. think about the backlash and absurdity. I have entered at least 100 times never been asked.