What's the problem with that?

Obviously, an ad volarem tax (https://en.wikipedia.org/wiki/Ad_valorem_tax) that is a tax that is proportional to the market value of a transaction like VAT wouldn't do anything.

But there are other kinds of taxes.

Eg fuel taxes in many countries are usually per litre, not per dollar of petrol. See eg https://en.wikipedia.org/wiki/Fuel_tax#Tax_rates

Carbon taxes proposals are also usually imagined to apply per kg of carbon emitted. Exactly because there's no prior price of carbon emissions.