If I buy a house for $100k, and next year some idiot pays $1M for a very similar house three streets down, did I just magically make $900k? Should I be taxed on that gain immediately? Should I be forced to sell part of my property to cover it? What happens when that sale occurs at a much lower price, due to my need to liquidate, did that lower the prices of all the houses in the neighborhood back to normal? Does only the first person to actually pay the tax owe the tax?
That's the reasoning we're applying if we tax unrealized gains on stocks (or any other asset). We take what the highest bidder is willing to pay for some tiny percentage of an asset, and assume that means everyone else could get the same price, yielding these theoretical valuations that have no bearing on reality.
Property taxes have a similar problem but that is a whole other can of worms. I'd love to live in a world where the local tax assessor is obligated to purchase your property on demand for 80% of what they say it is worth -- surely they would jump at the chance to realize an instant profit, right?
You simply can't establish value without an actual transaction. Without a buyer and a seller you are just making up numbers.
