"Free Trade" is like "Frictionless Wheel". It's trivially easy to talk about and is simple in theory, but in practice they both involve a lot of messy engineering.
For example, free trade in mayonnaise. That should just mean that anyone can import/export mayonnaise right? Well, no. Mayonnaise is food so there's certain health standards and facility inspection regimes. And you and I have different ones now they need to be harmonized. Also, what is Mayonnaise anyway? We need to harmonize our labeling requirements. Also. is your country producing subsidized eggs (major ingredient in mayo)? Well now we need to add a line to our contract that eliminates those subsidies.
> And you and I have different ones now they need to be harmonized.
Hum... No. Not at all. A company is perfectly capable of making mayo for different health requirements.
There are a few cases where harmonization is needed (mostly between small and poor countries), but overall "harmonizing" laws is just a propaganda presentation of giving exorbitant benefits for an external company that is bribing your government.
It is also about allowing a small player into the market. If there is no harmonization then then only producers who can afford to play the game are ones that can dedicate the resources to producing a hundred different types of mayo to meet various different local production requirements. Alternatively, this lack of harmonization simply allows the largest market to dictate the terms of the market for all other countries: consider situations where US states have different requirements for certain products and how this lets some states dictate terms to others, at times this leads to stricter emission standards when CA writes the rules ("yay" says the left) and at other times it leads to school textbooks declaring the US a judeo-christian nation and undermining the separation of church and state because TX writes the rules ("boo" says the left.)
As always, it is not as simple as it initially appears.
For example, free trade in mayonnaise. That should just mean that anyone can import/export mayonnaise right? Well, no. Mayonnaise is food so there's certain health standards and facility inspection regimes. And you and I have different ones now they need to be harmonized. Also, what is Mayonnaise anyway? We need to harmonize our labeling requirements. Also. is your country producing subsidized eggs (major ingredient in mayo)? Well now we need to add a line to our contract that eliminates those subsidies.
The lists of this kind of stuff goes on and on.