Also keep in mind that heated seats may not be the only option available. If you add in a few other options like backup camera, self driving/driver assist and maybe a few more you end up making a dozen or so different production lines and complex logistics. If you have a dozen features you are basically making custom cars at this point. It can definitely be cheaper to make a single model of car and lock features instead of dealing with all of that complexity. Sure, for one commonly purchased feature like heated seats it make make sense to have 2 production lines (at least for the seats) once you start adding dimensions to that matrix it gets expensive very quickly.