The tax on cars is largely for environmental/societal reasons. Pollution is an expensive, damaging externality for society largely caused by cars; the infrastructure for a car culture is VERY expensive to maintain; car-dependent cities are less attractive to live in.

A single-digit sales tax or registration fee does not cover these costs to society caused by car owners. Therefore, these costs must be covered by the general fund: from income, payroll, and business taxes. These all apply equally whether or not you own a car, so if you don't buy a car, you're still paying for the environmental cost of having one but not getting the benefits.

Denmark has done a smart thing by placing high taxes on cars. Reasonable taxation policy helps make this decision more rational. The flat tax usually makes sense because cheap cars are likely to be low-powered sub-compacts, which have a smaller impact, while expensive luxury cars are more likely to be high-powered, heavy, large vehicles.

But EVs (and hybrids, to a lesser extent) break this model. They are expensive, but environmentally friendly. It makes sense to encourage their adoption through tax breaks. That is not a shift of wealth from the poor to the rich, it's an intelligent exception in a model intended to shift costs away from all of society to the source of those costs.