I’ll again remind folks that Trump’s lying his ass off about the wall, and also that most of the wall is already built. There will never be a wall along all 2,000 miles, not when a significant portion of the border is the Rio Grande River and the treaty between the US and Mexico is specific about that and what that constitutes. If the US would build a fence along those sections, it would have to be on the US side inland - sometimes significantly so to avoid high water/flood levels, and that also means effectively losing ground to Mexico.

It would also potentially separate private land owners from their land that abuts the border.

It would also come into conflict with at least one Tribe that has land adjacent to the border.

Never mind that much of the unfenced border is in inaccessible and difficult terrain and would be easily circumvented by a rope (as Trump himself admits).

Oh, and most illegals actually entered the country legally but have since overstayed their visas. Majority aren’t coming as a result of crossing the border at places other than border control points.