This fact regularly seems to be forgotten by a lot of media reporting. People see "Musk is a trillionaire" and assume he has anywhere near that amount of liquid cash, completely ignoring that the company itself actually just lost $4.28bn.
Value is simply what the market is willing to pay for a share at the margin. As a founder, my own startup has a valuation, but that sure as hell doesn't make me a millionaire.
Oxfam has this daft 2% wealth tax idea on any assets over £10m. If we actually started taxing people based on paper valuations, most startups founders would be bankrupt by Series A.
Funny and related: https://www.youtube.com/watch?v=iHfJRON3b-w