We decide! There are lots of different taxes, e.g. payroll taxes. Most of them are financing taxes in some sense (taxing away funds that would otherwise have been spent), rather than distribution shaping, though all taxes to some degree affect both. 1/

We want financing taxes to be "fair" in their burden (always contested). There is some advantage to dividing financing across multiple smaller levies rather than concentrating them in one big thing (like a VAT), as that reduces incentive to game the one-big thing, makes evasion more complicated. 2/

Plus, taxes can have other purposes entirely, incentivizing or disincentivizing particular behaviors. For example, some people favor land-value taxes on the theory that it encourages more value (and usually density) in structures. 3/

Dividing the current role of income taxes into VAT + highly progressive, high-income, high-top-marginal rate taxes doesn't imply a position on those debates one way or another. /fin