Here's a few facts about WHY these things take a long long time...
- It's NOT a quick process. I'll explain a bit more about why down below. Here's the accelerated schedule for just a few RGV sectors.
- Even when the real estate is available, it takes a long time. Construction is not particularly slower for a 55 mile section than a 3 mile section!
- Just the planning and materials acquisition is half a year;
- Custom design is almost half a year
- Real Estate acquisition is half a year to over a year... and that's quite optimistic
(This is from "Levee Wall Overview, Border Infrastructure PMO", Bob Hardbarger 28 April 2017 US Army Corps of Engineers
- Politicians talk about "The Wall". Actual CBP border people, and those responsible for construction and use, talk about the Border Wall System. It's not just a chunk of concrete with metal on top!
a) In the area MOST impacted, the LRGV (Lower Rio Grande Valley), there's an existing levee that goes back at LEAST to 1915. Bringing everything up to modern standards requires at the least improving the levee to match FEMA certification requirements. Here's another diagram (same source as the above, page 6 this time:

b) Here's a diagram describing the overall Wall System. I've seen the RFC's (Requests for Contract) ... the fancy FODS (Fiber Optic Detection System) had to be installed... yet actually lighting up the detectors is a whole separate process taking still more time. Just consider what's involved in spec'ing and building this much (particularly w/ careful environmental etc assessments along the way. They were allowed to overrule many complaints but they still had to do everything possible to make it as good as possible!). Same source (because it's handy ;) )

NOTE: I just did an intensive search for these documents online. I have a whole collection. Amazingly, they have been removed everywhere that I can find. That includes the place I absolutely would expect to find the one referenced in this post: the USACE Digital Library, which includes responsive FOIA documents, of which this is one! There are lots of older documents, but NOTHING on this subject at all. Isn't that interesting!