Making the batteries smaller individual containers would absolutely add a lot of weight which is already a massive problem for EVs. One large battery pack acts as a structural component so breaking it up not only means adding additional weight for the additional casings, it also means eliminating battery capacity and also adding brackets to structurally attach them to each other (which is both heavier and weaker than one large casing).

This is also an odd thing to nitpick. Battery cells themselves are modular and different manufacturers are using like cells depending on the origin of those lithium cells. This is not unlike battery packs in things like power tools, those battery packs are just containers around cells like this: https://www.samsungsdi.com/lithium-ion-battery/power-devices/power-tool.html

These 3.6 or 3.7 volt cells are more or less the same across all brands (Dewalt, Milwaukee, Ryobi, etc) and if you wanted to you could open those batteries and replace cells. Similarly, people can and do open up EV battery packs to replace battery cells. So long as you k ow what you are doing when you rewire them and don't kill yourself touching a large capacitor they are repairable and replaceable components.

Are you just as upset that you can't for example, swap brake pads between cars? Or intake manifolds? Or that there are a million variations of a spark plug?