Uuuuummm.....not true; my son's '03 Dakota and my '07 Ram have ground straps from firewall to engine; the Dakota has 3 or 4 total! I bet I'll find some on my '09 Challenger if I go look. And my wif'e's '93 Caravan had 1 or 2........(I am talking about braided straps not some flat copper strip, BTW..... yes, they are plated).
And the flexing on most starter cables is over a 1'-2' length; firewall to head straps that I see tend to be a lot shorter and it is the movement versus length that is critical; short lengths obviously get a lot more degrees of bending per inch.