My 2015 Chrysler Town & Country has heated seats. By pressing the switch, I can choose heated seat and back, or just seat. Why wouldn’t Chrysler have gone one step further and installed a switch that offered back-only heat as well??
Because the heated seat is for comfort, not to treat a bad back.
Because the Chrysler finance department won the argument over the design ergonomics staff.
Probably not the steel cage match. That is reserved for really important things. 
That’s an extra cost upgrade for the next model year.
It wouldn’t be hard for this to be installed by someone who does electrical/audio work on vehicles. It might mean some aftermarket switch(es) but is quite doable.