The difference is mostly timing. If I bought a house for $150,000 in Jan 2020. And the appraised price of the home in Jan 2021 is $171,000 (+14%). I did not make 21,000 last year unless I sold the house. My current net worth went home. But I did not make that $$ until the item is sold. At least, IMO.
Further, as you mention earlier re: untaxed assets, the US tax system is based primarily on taxing cash exchanges. Sales tax, Income tax, Business tax based on revenue et al. Except for property tax, there are not any other taxes, that I know of, that are based on the value of an asset. I know Elizabeth Warren was pushing for a change. But that does not exist currently.