>>The gain is not realized until the asset is sold.
Some of the largest fortunes include massive assets that are tied in trust and never will be sold. But the asset can be borrowed against to purchase more assets. So the wealth multiplies itself at a 0% tax rate or 15% of the capital gain if they ever do decide to sell something and live on the proceeds. If it is rational to minimize tax, this is exactly what the current system is structured to incentivize and it benefits those who can accumulate the most over and above their own costs of living and not draw a salary.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
