The point of owning more stock (or wealth in general) is often not simply "living comfortably" or any sort of pure consumption. If that's your framing, lots of large-scale economic behavior will remain unimaginable.
Similarly, it's not "just a rat race to show off", either – though there's some of that, as a psychological motive among the very-rich. That's still not sufficient to describe the logic of what's actually going on.
Rather, ownership of such highly-valuable assets is about the control it offers, over how those assets are used. In this case, the primary asset is "Zoom Video Communications Inc", and all its intangible & tangible assets, including experienced staff, intellectual property, brands, relationships with locked-in users bases, etc.
If Eric Yuan wants to guarantee he retains the largest influence over what Zoom does in the future – because he has pride of creation, because he believes it's a valuable service for society, because he finds it an intellectually stimulating way to live life & exercise his unique talents, moreso than just being a 'consumer', he must retain the lion's share of equity ownership. Such ownership is how our society tracks control over big synergistic flotillas of resources.
The theoretical amount of lifetime-consumption that ownership could instead represent, if sold, is a tangential detail to the full significance of the holding. And, a true plan to "sell it all" would likely cause much of the value to evaporate – as some of the perceived trading value of the circulating shares is premised on the founder's continued immersion in the business.
To a limited extent, the ability to liquidate "a little", & thus get the lifetime-consumption-security that you've portrayed as the whole "point", is relevant – but not because it completes Yuan's ambition, ensuring a lifetime of 'comfort', but because it could allow even more focus on the bigger social enterprise involved.