system
A chat on subreddit /r/wallstreetbets
human
it's what tesla uses for their car batteries... basically multi-generationally old tech, terrible fill factor. The future is towards specially-shaped-to-fill-space batteries, ala Apple laptop batteries.

gpt
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.