Movie "Sicario": why dead bodies in walls of house?

Agree with you on the dumping in public. AIUI, the cartel is, well, gunshy about killing in the US. Isolated notorious examples aside. (E.g., the hit on a cartel lawyer/financier in an upscale shopping center in a Dallas Fort Worth suburb: https://www.nbcdfw.com/news/local/Murdered-Southlake-Man-Headed-Major-Mexican-Drug-Cartel-Attorney-Says-374504041.html) See also, the difference in murder rates between Juarez and El Paso, or any two twinned Mexican/U.S. border towns really.

If they didn’t want to hire the pozole guy, and his procedure doesn’t look all that hard, then backhoe a nice hole somewhere and dump. Or do what the House of Death did for their victims. What that is, I don’t know, but I imagine an antelope hunter will find something grisly near Fort Bliss or the Hueco Tanks, one of these days.

Borderland Beat (http://www.borderlandbeat.com/) is a great resource for news like this, and if you find yourself currently plagued with easy, dreamless sleep.

Is it an American-style wall? Adobe or bricks can easily be half a meter thick (exterior brick walls are usually double, adobe will be single).

Typical US home construction uses stud walls with a “2x4” at their core. The 2x4 is actually 1.5 x 3.5 inches. The stud wall is then faced with drywall (“sheetrock”) that’s 1/2" - 5/8" thick. This is the sort of construction that was shown in the movie.

Exterior walls might use 2x6 or 2x8 studs (to allow for insulation), but even the latter only creates a gap that’s 7.5" deep, not quite enough for an adult human head.

It’s a thing that has happened a few times in real life. A notable example is British killer John Christie, who concealed some of his victims in the walls at 10 Rillington Place.

The bodies would leak, too. Places where you expect corpses to be stored take a great many pains to keep the liquids and odors from offending, with imperfect success.