Why a Long Wall rather than a Short Ditch (or Dry Canal?)

Thinking about this wall obsession of Trump's (and the media) it occurred to me, how about the opposite of a wall, a deep ditch, like a moat? Then, looking at a map, it occurred to me that our northern boundary with Mexico is about 1930 miles long while Mexico's Southern border with Nicaragua is only about 530 miles long. So why haven't either the Trump or the preceding administrations proposed the idea of working with Mexico to place a wall along Mexico's Southern border (1/4 the length)? Surely it could have been cheaper and more secure to control over land migration there. Seven "dry canals" have been previously proposed to create a second channel to increase trans oceanic shipping. A dry canal is essentially 2 ports for ships to bring cargo to offload containers on 2 tracks of a railway. A cargo is then sent by the railroads to the other ports and reloaded on other ships to continue to destinations. Dry canals 1 and 2 are north of Honduras, one being only about 300 km. and the other 280 km. long. The countries in which these dry canals are proposed might welcome international development of two ports and a dual railroad. It's getting expensive for the biggest (Panamax) ships now to go through the Panama Canal and bigger ones are being built, requiring more locks. Surely 280 - 300 miles of double railway carrying valuable cargo would be easier to guard than 1,900 miles of U.S. border. So why haven't we heard anything about these ideas as alternatives?