They are assets from personal perspective yes. But they are in the form of equity in company's balance sheet in which it is invested. Which is why it is on the liability side of the equation (A = L + E).
If you impose a wealth tax on this, which side are you imposing it on? Company or individual investor in the company? You cannot impose a tax on what is technically a liability on the company's books. So you would have to impose on the individual investor. Lets try with an example: assume the wealth tax is 10%. Lets also assume I hold 5 million common stock, pre-seed, in a startup I founded (45% equity). How would the stock be valued? It should be zero. So I pay zero wealth tax. Then I get an investor onboard, who invests say $1 million at 10%, valuing the company at $10 million. Lets assume, because of this capital infusion, my equity gets diluted to 40%. That means I am on paper worth $4 million. I immediately owe 10% of it or $400,000 to tax authorities. Without having made a single cent in revenue. My co-founder would also owe $400,000. We have no money of our own except to use the investment money to pay the taxes. That is $400,000 + $400,000 = $800,000. We are left with only $200,000 in the bank. Before we even started the company.
You might say 10% is too much. Lets reduce the wealth tax to say 1%. Or even 0.1%. Then the entire idea becomes meaningless. Might as well just charge extra corporate tax.
If you impose a wealth tax on this, which side are you imposing it on? Company or individual investor in the company? You cannot impose a tax on what is technically a liability on the company's books. So you would have to impose on the individual investor. Lets try with an example: assume the wealth tax is 10%. Lets also assume I hold 5 million common stock, pre-seed, in a startup I founded (45% equity). How would the stock be valued? It should be zero. So I pay zero wealth tax. Then I get an investor onboard, who invests say $1 million at 10%, valuing the company at $10 million. Lets assume, because of this capital infusion, my equity gets diluted to 40%. That means I am on paper worth $4 million. I immediately owe 10% of it or $400,000 to tax authorities. Without having made a single cent in revenue. My co-founder would also owe $400,000. We have no money of our own except to use the investment money to pay the taxes. That is $400,000 + $400,000 = $800,000. We are left with only $200,000 in the bank. Before we even started the company.
You might say 10% is too much. Lets reduce the wealth tax to say 1%. Or even 0.1%. Then the entire idea becomes meaningless. Might as well just charge extra corporate tax.