It forces them to unify everything except VAT (and land tax). Or, and I only realised this just now, do something complicated with imports, customs, and how to handle cross-border sales of digital goods (the EU does this, but it is a mess).

If I had to guess, I’d suggest haven’t already done what you suggest for at least one of these reasons:

1) because it’s economic mutually assured destruction

2) because tax codes are long and complicated (and sometimes deliberately written as political favours), and they’re as afraid of touching it as PayPal are of touching whatever Musk wrote 20 years ago.

3) because businesses that have revenue (Apple, $229bn) comparable to the GDP of the nations whose tax systems they are exploiting (Ireland, $293bn; Luxembourg, $59bn) is novel and governments are slow to adapt.