Jim, it is because it is so vital that it can be used to try and stop Brexit. I think that is what is happening.

If a free trade deal is agreed, there is no border problem .
If not, the volume of trade across that border is very small anyway.
UK says it can be left open without significant problems.
I believe them. UK have no reason to lie. EU does. They do not want Brexit.

It would be utterly impossible to monitor all the 200+ crossings anyway.