Did I say mandatory? I meant optional! You’re “free” to die in a cardboard box under a freeway as a market capitalist scarecrow warning to the other ants so they keep showing up to make us more!
They can be used as collateral because they are assets that have value. You can use your car or house as collateral too, and neither requires payment of federal income tax.
There isn’t a federal tax on most assets. It’s income that’s taxed. If your assets gain value they can be sold, at which point you pay taxes on that income, though often at a reduced rate (e.g. Capital Gains Tax for selling stock at a profit).
Except most state/local governments do have property taxes on houses, land, and cars. Not unrealistic to apply the same towards other assets. Specially since taxing homes and cars is counterintuitive because you’re taxing necessities, while taxing monetary/investment assets like stocks would make more sense to encourage more spending instead of just hoarding the money.