The EU and US standards are very different and products for one can’t necessarily be shipped to the other and vice versa. Examples for food include massive differences for colourings, preservatives and the like. Europe will not accept chicken washed in chlorinated water or bright froot loops. Health traffic lights are also going to be different.

While it’s possible to have 2 production lines, 1 for each economic zone, that’s expensive for producers and shippers.

A full renegotiation of the US Canada trade border would be chaos

But that is precisely what Trump is demanding now. Even though he tore up NAFTA and renegotiated the deal in his first presidency (I am sure I remember him saying it was the greatest trade deal ever signed and the greatest win for TrumpAmerica).

They don’t need to join it as a full member to have trade and economic benefits. Simple (!) alignment with standards and regulations will allow free access to the market and free movement of people.

Here’s a video with a lot of guidance and examples of use: https://youtu.be/EAQI2ZSmxPU?si=lF5xr8fKyCRHEk7F