system
A chat on subreddit /r/wallstreetbets
human
They perfected the form factor and can make large runs. I think this is always beneficial in the long run.  I think they were leveraging the fact that consumer electronics already uses them in mass. Anything custom is more expensive.

gpt
the form-factor is non-ideal, and was perfected long before TSLA came on the scene. (18650s have been a thing for a while now.) Agreed re: custom being expensive but at the scale involved, it would be relatively minor expense that would eventually be amortized and ultimately result in better product. Meaning, esp. if you are building a $5b factory, build it to make something more ideal than 18650s.