You might be OK with being taxed in the form of some of your stocks, but would you want to be taxed partial ownership of your real estate based on increase in its market value? What about private high-risk equity in a venture you started by risking all of your own money? It could go to zero (or millions) in a decade, but for now the government becomes an increasing shareholder because of its current YoY volatility...

Taxing non-income wealth increases by any means incentivizes the government to manipulate the prices of those assets -- and more than any other entity, the government possesses the power to do so through regulation and legislation.

It also makes the government the eventual largest capital owner, which could create all sorts of unintended consequences. It could sit on those assets indefinitely (like it does its liabilities) instead of selling and using the money to fund itself. There's a special kind of poverty that exists when the government owns almost everything.

Also, there's a big difference between legal worth at a given time and fungible value at that time. The difference (or lack thereof) between the two could itself create all sorts of opportunities for gaming the system when fungible value becomes a basis for being taxed.