I think it's because they people you normally think of as "wealthy" don't have most of their property in the form of house or car - only the poor or the shrinking middle class that's the case.

Car and land cost money for the state, you get a "direct service" from your tax. And wealthy people have lower rate (compared to their revenue and wealth) than the average Joe

Here we talk about a tax without a "direct service" associated... but that would be paid only by the wealthy, not by the average Joe