This is simply incorrect.
To make the tax incidence on wages and capital gains equivalent, you must first deduct losses due to inflation and risk. For wages, inflation and risk round to zero. For long-term capital gains inflation and risk are large and often the majority of the "gain". Short-term capital gains are already taxed like wages.
In the US, unlike some other developed countries, there is a very limited ability to deduct losses due to inflation and risk from long-term capital gains. Consequently, if they made the tax rate the same as wages then the tax incidence on capital gains would be much higher than wages.
As a policy matter in the US, they fix this large difference in tax incidence by reducing the tax rate instead of adjusting the cost basis for inflation and allowing full deductibility of losses.
If you pencil out the implications of these two policies, I suspect you'd find that you like the way the US does it better. Making risk and inflation deductible to equalize tax incidence enables a lot of financial structuring.