Individual cells might in theory be modular, but Tesla for example is gluing them to their battery packs which makes them much harder to reuse at the individual cell levels. The battery pack it’s self is becoming a structural element though it can still be removed, but unfortunately it’s proprietary.

On top of this EV batteries are getting very close to lasting the normal lifetime of the vehicle. Car manufacturers are likely to tray and leverage this as a form of built in obsolescence by making replacing the battery unappealing under normal usage patterns.