Probably because it costs more money and nobody will pay for it. That's the best guess I have.
Personally I solved the problem by buying a lithium ion car starter for both of our cars. Works great, but I really would prefer the cars just had some redundancy built in.
I have been contemplating using the second one as an auxiliary battery after installing a fridge in the boot, though. Basically a matter of fitting a voltage monitor and a hefty relay.
Leaving the headlights on (~10A) as you leave the car in the evening will still let you start it in the morning, for example.