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.