What happens if you fall under two different tax-breaks or if a payment was taken on a certain date which falls before or after a cut-off point? A lot of complexity, that the developers could solve but will not necessarily know how to handle. At least when you submit this to the relevant tax authority, they can spot alarms like this and decide in a reactionary way how to handle it, adjusting tax codes or providing rebates.

There are just way too many things that change too quickly.

That said, I am all for simplification but then you wouldn't need open-source software, you could just do it in a spreadsheet ;-)