1. Sourcing is difficult. Would you buy some random batteries from China? Do you trust the dealer that it won't fry your customer in few months after installation? Can you verify that what they are selling is what you have in your hand?

2. You need to know a lot about batteries and even more about battery management system and then know how to integrate batteries + BMS into a car. Sure you might be able to reuse existing BMS for a car you are targeting but it will quickly become a bottle neck in your manufacturing if you do so.

3. Pairing of parts. Related to point 3. You might be able to build a battery pack, are you able to also force the car to take the battery pack? Car might outright refuse to talk to it, because it is not paired and OEM won't let you pair it, because "safety", but more like safety of their profits.

4. Shipping of Lithium batteries in general is huge PITA. Try to ship battery of any size with Fedex/DHL/UPS. They either outright refuse to or you will pay a lot of extra.

I saw a teenager on YouTube replace the battery on an early LEAF with a newer, higher capacity pack from a later model and (separately) a Chinese firm offering replacement packs for the same car at capacities never offered by the OEM.

Notably the kid said the hardest thing about the swap was sourcing the replacement battery.

Swapping for a used battery is pretty easy if you have a lift. Far easy than swapping an ICE engine. It makes sense that there's more aftermarket batteries in China as they have far more EVs and willingness to tinker. We in the US have become cultural allergic to tinkering, everyone cries that you're voiding the warranty or it's too dangerous.

Rich Rebuilds used to do stuff like this too.

I really think given a few more years when millions of Model 3s/Ys reach their 2nd/3rd owners, we will see this become more routine.

Also people talk about pack replacements like they are a single monolith but many cars have packs composed of 10-20 modules. Some modules can even be repaired at the cell level. So costs can trend down over time.

The trend has been towards unrepairable monolithic packs. You could do module replacement in the model S packs but since model 3 it's all full of foam.