There are lots of people here making good points about safety but there are many reasons as to why EVs are built the way they are. One of which is just physics.

TLDR: Sorry this isn't short, batteries and stuff aren't easy to ELI5 but I think this works? (Other people feel free to correct if this is bad)

Batteries store a potential energy difference - this is a tiny amount - need lots to get correct energy.

After correct energy is achieved, tiny capacity = no range = stack EVEN MORE batteries.

Wires create resistance - loss in energy = bad. Modular connectors = more wire = bad.

Make pack big single block = less wires = less loss = more range = good

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First, some battery background. A battery stores energy, for a simplified example: If I hold a bucket of water over my head, with an empty bucket at my feet, there is a potential difference in energy between the two buckets. The water in the top bucket wants to go down to the floor, and I will catch it in the bottom bucket to reuse it/recharge.

This is our full charge, our maximum voltage/difference, when all the energy is ready to move but not yet moving.

I pour the bucket out and all the water is at the bottom bucket and none in the top, the water cannot go down any further and we have no voltage/potential falling energy**.** The time it took for the water to flow down would be our capacity. (very simplified)

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The energy is very small and we need lots of these together the create the required high voltage 300+V sometimes. (This is wired in series) However, the time it takes to pour this out does not give us that power for very long, we combine that first block so we have many buckets pouring one after the other (this is wired in parallel)

We need these modules to be an exact size of series to get the car to run, and a size of parallel to give our range. Some cars are built like this in the factory, where each module is separate. (For an EV I helped design and build we need nearly 100 individual batteries in series to get the movable energy, but this would give us a range of only a few kilometres so we stack 50 of those 100 battery modules together)

Then we get into physics and materials. Engineers use the modular method to design and build the packs, but this is why they might not be modifiable after they are built.

Electricity is most efficient when it goes short distances, if you have to run a long way you will be out of energy by the time you get to your destination, this is like electrical resistance. When building an efficient car we don't want long distances of cable. If I want to swap my battery I need to make all the modules separate, i then need to run lots of wires to a single point and then have a big High Voltage connector to join them to the other parts of the battery pack and to the entire car. Multiply that by 100x and you have lots of metal in the way of our flowing electricity causing resistance and taking away our power as heat instead of spinning wheels.

EV designers make the packs as much as one piece as possible to stop having connectors and resistance. Also the more parts that move and rub together are just more parts that can fail and need to be replaced. The best part is no part. So combining everything into one solid brick is more reliable, safe, easy to manufacture, and also more electrically efficient. This is one of the many reasons as to why some EVs have so much better range than others with the same size battery on paper.

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I hope my examples were understandable and somewhat accurate. There is a lot of advanced engineering that goes into making finished products like cars and adding new technology makes it even harder to explain/understand!!!!!