from what I understand it's because it's a lot more expensive than its alternatives.
Like yes, for a bunch of structures you can neatly automate it (see most rocket production), but the shapes of (current) cars don't easily offer themselves to similar options. Automation is possible but would probably be finicky and require a lot of space and energy (for the heating).
but someone else please jump in if you know better/more.
Because it doesn't make a lot of sense to invest heavily into saving 100kg on the frame and then adding a 500kg battery. The i3 had a small, lighter battery and an extender.
The range killer is really drag, not weight. Weight contributes, but at high speed drag is such a large draw that nearly nothing else matters.
The rolling resistance coefficent of a car tire is about 0.01 and the force grows linearly with mass. Drag is v^2 and the coefficients are more like 0.2 - 0.3 of the frontal area on most EVs.
Weight savings don't offer that much range savings so there isn't much incentive to pull weight out of a design, especially when carbon fibre tub construction is so much more expensive.
BMW made a bet batteries would remain very scarce and expensive, a bet they lost pretty throughly.
The BMW i3 had a carbon fiber frame and was still reasonably priced back in 2013, yet no other normal cars seem to have went this way.