The most important trade barrier often used in the free trade areas that are non **customs unions ** is the “non tariff barrier” which is using the national level inspection rules, regulations and laws in a way discriminatory against
It turns out in the real non theory world this is very common - in fact you can see in the inter-arab trade among the theoretical “free trade zones” (Maghreb, etc) that very little common trade occurs due to the non tariff barriers - the tariffs are gone but they use the other customs rules and products regulations and rules to exclude the goods - the Moroccans exclude the Egyptians, the Egyptians the Moroccans etc.
The same can be seen for the West Africa, etc. -
So why? Because centuries of experience show that they do not work without the common standards.
Although the other responses have focused on the idea of the currency manipulation, this is not really a serious motivation as that can be avoided via the exchange regimes - the set rules on the exchange rates, which in fact existed prior to the Euro for the major EU currencies.
As a matter of achieving a common market, a common currency is very helpful to reduce the transaction costs to the flow of the goods at many levels from the retail level – it is a reason why you have a common national currency and not multiple state currencies!!!
If you stop"wondering" and take a look at how the trade exists between your own internal “United States” many of your wonderings will be resolved. Even in the 18th century when your founders were writing their constitution, they were aware enough of the issues in Trade that they made your famous Commerce Clause as it was already in the 18th century well understood that besides Tariffs these rules were easily used to impede trade, to favor the home state over the neighboring state, etc. The common market then breaks down and does not in fact function.
And since outside of the feverish imaginations of the American extreme right Libertarians there has never existed an organized non-civil war country where there were not some levels of regulations on the quality of goods, on the rules of services - as in the real world - outside of the bad novels - the real human beings do not accept wild wild west conditions - standard rules setting on the goods and the services is an absolute necessity for a common market, a customs union, that actually functions.
Encore, these are lessons that have been understood since the 18th century, it is not a new lesson.