I don't understand why this is such a big deal. Many countries around the world have a hard border with a physical barrier in place to stop illegal crossing/smuggling. It doesn't seem inherently unreasonable that the US has one as well, especially given the border with Mexico is well known for illegal crossing and drug smuggling.
Why is this seemingly normal function of government seen as such a contentious issue that it's worth shutting down the government for, and what do the Democrats have to gain politically from the continued illegal activity on the border? Surely Democrats are equally affected by the criminal behaviour as well(?).
They could make a deal and get something that would make a real material difference to the electorate, and I don't see why they're going to the mat for this.