Back to the topic at hand....
The question shouldn't be 'why hasn't he folded', it should be 'why hasn't he used the tools at his disposal to accomplish what he needs to'.
We don't need negotiations, we need a project manager who evaluates the tools and resources at his disposal and uses those in the best way to achieve his goals.
For example, as many have pointed out over and over, he can declare an emergency and use the military and the corps of engineers to, in the very least, start work in high risk areas. Declaring a state of emergency isn't as big of a deal as some make it out to be- hell, Obama did it over a flu outbreak.
As for the money, I bet most didn't know that the President has over $100 Billion in Presidential Discretionary Funds for ongoing operations of departments under him.
https://www.thebalance.com/current-us-discretionary-federal-budget-and-spending-3306308A good project manager can look at the funds he has access to, and I bet would be able to shift around enough to get what he needs where he needs it.
That's just one of several ways he can accomplish this. He may not get exactly what he wants the way he wants it, but it would get the ball rolling and actually get something done.
So let's go back to 'Why'. Is he not aware of this? Does he not want to go this route (which is another Why). Maybe we should ask 'why' he is playing the politics game instead of using the tools and resources at his disposal.