...so that NI would remain part of the EU customs union and could harmonise trade with the Republic, but also still remain legally within the UK with a border which would be actually possible to police.
However, the DUP won't accept any divergence from "mainland" UK, even though policing, power, agriculture, social laws and, well, pretty much everything else in the province is completely different already.
As DT says, the EU has to protect its borders.
And so do we. We don't want an open door to all those highly-qualified EU plumbers, scientists, vets and health workers meeting our skills shortages and propping up our economy, society and NHS, do we...?
On a more serious note, as explained, an open border would lead to a criminal/smuggling explosion because of the differing SHE standards/tarifs/etc etc between the UK and the EU affording opportunities to organised crime to make massive profits.