The reasons power cords are not made from solid core wire are safety and ease of use. Have you ever tried to bend a 12 awg wire....10 awg? I believe the safety thing is the concern that frequent bending of a single solid core wire could lead to fatigue cracking and a higher incidence of wire failure. Romex in the wall is fixed and not subject to repeated bending. One of the manufacturers who posts here, has said Romex would actually make a high performing power cord but it would not meet electrical codes for that purpose.
I have made a couple of 7 awg unshielded power cables using six strands of old stock WE 10 awg wire (the same stuff used by TWL in their American series). I use them from two dedicated 20A circuits (wired with 10 awg Romex) to 300 wpc Class A monoblocks and the DIY 7 awg cords sound every bit as good as expensive IsoClean or Furutech power cables I also have here.
Some day I will try the Romex thing.
