For an EV the main design constraints are aerodynamics and weight. If you need to add additional reinforcement to make up for the battery packs that haven't been installed, you're carrying that weight around all the time and taking a range penalty.

So assume your vehicle has 3 packs. You normally run it with just one pack because that's all you need to commute and run errands during the week. When you go visit the relatives in Florida, you install the other two packs to get the additional range. The car needs additional structural elements to meet safety standards when the extra packs aren't installed and they add 5% additional weight. That's weight you're carrying around every day, and that has a range cost.

Could it be worth it to swap out the extra structure elements when you install the extra packs, so at least you aren't taking the range penalty while driving long distance? Yes, but the manufacturer isn't going to want the liability of a customer removing what is a passive safety device, and then forgetting to reinstall it when they go back to city travel using only one pack.

NHTSA will also have input to this. From what I can tell - the applicable FMVSS is this one that deals with EV side impact protection, electrolyte spills, and electric shock.

https://www.nhtsa.gov/document/final-rule-occupant-protectio...

If you want the batteries to be removable, I suspect the extra couplings in the water coolant loop would make that problematic.

Check out how tractor hydraulic attachments handle liquid couplings. And hydraulic systems work at much higher pressure than simple coolant loops.