The vast majority of personal tax paid is income tax, which is very adjustable and the most powerful lever the government has to change what people do.

If most tax came from things that aren't "leverable" (everyone pays the same VAT/sales tax on a given item, they don't pay the same income tax) then it is forced to be much simpler.

Also, a decent amount of "social program funding" comes via things like the EIC and other "tax credits" in the US vs direct payments.