Assets are real, but not the valuation of them. This sort of basic error, which seems to only appear on the "tax net worth" side, is good evidence that this is not a reasoned position, and thus you will not be reasoned out of it.
Valuing assets is a routine, widespread thing.
You can't just handwave away the fact that the real world exists.
A painting can be valued. A vintage car can be valued. A movie script can be valued. My house's value changed a bunch during COVID; I'm still taxed on it! Each is an asset; none of these examples can be perfectly valuated; each may see its value change dramatically over time, but you can still estimate its worth pretty well.
Hell, I can put my house address on Zillow and get an estimate in real time.