What political aspects prevent land purchases in North America moving national borders? [closed]

The border between the US and Mexico is about 1,952 miles long. The current US president, known for making real estate deals, previously wanted to build a large wall along the entire length of the border, a very costly prospect.

Depending on the costs of North Mexican real estate compare to the costs of a border wall, it could be cheaper to do something like the Louisiana Purchase or Alaska purchase, down to a point where the border is shorter, or otherwise tweak/smooth out the border to reduce the costs of constructing a wall. This could be politically popular in the US, and overpayment for the land could be accompanied by an announcement that Mexico would pay for the lower-cost wall (from the proceeds of the overpayment).

Who all would need to be convinced in order for such a deal to go through, and what other political obstacles would have to be overcome?