I assumed they were a poor fit for the starter-motor problem, the need to supply super-high current (load) to get static forces overcome, swing the engine into it's cycle. But, I see they are capable of that burst of high load. So, indeed it begs questions.

I suspect the engineer approach would still be to demand a brick of standard lead-acid battery size, to fit into the hole, and the least possible change to wiring harness and associated behaviour.

Building out cars which need a different box in the hole would be something you do with supreme confidence it isn't making you awkward in the marketplace. If you wind up at the petrol stop in death valley with a dead battery, and alas a good ol' lead acid unit can't fit, that isn't cool. Battery replacement happens in all kinds of places.