The structural needs of each car are different. If the battery can handle them all, then it is overbuilt and you can save enough by not being swapable to make your car cheaper (or give it longer range or some other trade off) so you won't do it. If you have a different battery for each car you can follow your plan, but then the logistics of handling all the different batteries means nobody has the right one for your car in stock.

There is only one place to put a large battery pack. However there are lots of places to put a smaller battery back. Or you can have a non-rectangular battery pack in that one place to better fit the room you have. Cars are about compromises, so a standard swapable battery pack will be smaller than the customized for the car one you can make. A tiny economy car doesn't have as much space as a large family car so plan is harmful to families that want to go on vacation as they have to stop more often.