[jlokier]

In fact the UK has something like that because of progressive taxation.

The idea is that a tax system should be "progressive", meaning people who earn a high income should pay a higher percentage of their income in tax, and conversely people who earn very little should pay a low, zero or even negative percentage.

Now imagine you work for two employers overlapping in time. E.g. one job in the daytimes and another in the evenings.

Your employers don't know about each other, so your total earnings before tax will not be known by either employer, but you should pay a higher percentage of tax due to your higher total income. So the state has to be involved in the calculation.