Now think about how they're going to respond to it.
A major problem with taxing unrealized gains is how to measure them. For publicly traded companies that's pretty easy -- the stock is undergoing regular market transactions so you have a pretty good idea about the price. But what about assets that aren't? Closely held private companies that aren't listed on an exchange and haven't undergone any stock transactions in ten years. Art. The value -- or liability -- of a private contract for the future sale of goods at a defined price, when the market value of those goods might have since changed, or depending on what they are, be indeterminate.
It creates endless opportunities for playing games, and that complexity is exactly what allows the people who can afford fancy accountants to pay less in tax than everybody else. If you want to fix it you need to make the system simpler rather than even more complicated.