Income from capital has a low tax rate in the US (and many other countries) because you can deduct neither losses due to inflation nor losses due to risk, both of which are substantial for capital income but non-existent for wage income. The lower tax rate is simpler than actually accounting for these differences.

Treating wage and capital income equivalently would require recognizing losses due to inflation and risk that simply don’t exist in a meaningful way for wage income. Taxing them similarly without very negative consequences requires recognizing these differences in some fashion.

Taxing wealth has myriad additional problems. In the US, about 2/3 of wealth is completely non-liquid so any theoretical valuation is fiction and highly leveraged.