In the US, there is the fiction that FICA pays for your Social Security pension, somewhat proportional to your contribution, though high contributors don’t get benefits in proportion to contribution. There is a legitimate fear by the government that if it becomes viewed as just another income tax that political support for the tax will evaporate. It is really important for it to be viewed as a pension contribution of sorts, which is obviously capped by how much it pays out.

If you did not limit the taxes and they explicitly became income taxes (in appearance), many upper-middle class Americans would find themselves paying upwards of 50-60% taxes on income for vanishingly few benefits. That has political ramifications that the government would rather avoid.

The challenge for the US is that the taxes on the middle class are very low relative to e.g. Europe. But they cannot squeeze enough taxes from the top 20% that pay most net taxes to make up the difference.