Technically the bracket system fits the definition of a mathematical function but taking your intent, I think a single function is actually less flexible, by introducing brackets you can specifically raise or lower the tax burden on demographics that have been shown, at least in America, to form meaningful statistical cohorts.

Plus the wealth percent based rate creates a direct negative impact to wealth accumulation, meaning that rich folks will likely be trying to offload wealth to keep their rates down, meaning that wealth can go back into the moving economy, meaning into the hands of the people it has been horded from through wage theft and price gouging.