That’s how battery chemistry works. Even this, if it is real, is a bunch of individual cells in a bank. There is no alternative; you can’t have sufficient reactivity between dissimilar materials to generate the types of voltages required in a single cell. You need multiples of them in series to hit 200 volts, 400, 600, whatever is required by the vehicle’s drive hardware.

Batteries, being containers for chemical reactions, are subject to the core concepts of chemistry. Namely, that increased surface area increases the speed of the reaction. You could make one enormous battery instead of multiple smaller cells, but you’d never get it to discharge fast enough to make it functionally useful.

Like the other responses, the battery chemistry and design voltage are the major reasons for cell sizes, but also, smaller cells means they can be isolated if necessary.

In a Tesla battery pack for instance, each cell is connected by a single small wire that also doubles as a fuse. If there is an issue with the cell the wire will heat up and break. opening the circuit and separating that battery cell from the rest of the pack. This also means that a failed cell doesn’t take out a significant portion of battery capacity. Other manufacturers do this as well, but not all, and some implement similar capability in other ways. This method functions as a sort of “passive” option since it doesn’t require the BMS to make a decision to remove those bad batteries from the pack, physics just does it and the BMS adjusts to compensate when the cell no longer is connected.

Good battery management systems that handle things like charge leveling individual cells, can mean the difference between batteries degrading noticeably in a few years and the pack as a whole lasting a decade before that noticeable degradation. There are a lot of poor battery management systems on the market, EVs are no exception and if anything they make this issue more noticeable because of the increased usage.