Because those stock grants are conditional on a lot of things. It's not money yet, it's potential money if certain things happen. Yes, those things are likely easily achievable - they either unlock over time, or on the IPO, as long as he is still with the company. But until they vest, they don't fully exist yet.

FWIW, when they do vest the IRS will consider the value of the stock on that day as income. They don't mess around with that shit. Even if he decides to hold the shares, he will still need to pay taxes on them based on their value on the day of vesting, but not before. He has people to advise him on what is best to increase his treasure horde, though.