I know there are workarounds you can do yourself but why don't new cars come with them. There's redundancy built-in to many things.
I remember a long time ago a battery with a switch. Messy so why can't there be an internal switch? As Anker show you can get enough to start the engine from little capacity, so a big battery and a small battery built-in to a new car and if the bigger one is knackered then switch to the smaller one. Then during driving the bigger one gets topped-up first and then smaller battery and then you're always with two batteries.
Off to work tomorrow on my bike, the one where if I start the engine and it doesn't start then a super hot cup of tea fixes it. Cleaned the chain and pumped up tires and it is going to be very hard for it to not work (assuming hot tea).