What's Apple's incentive for having two different asset pack formats? It seems like more work to support parsing and generating both, and Apple expects you to use their tools to generate asset packs.

Working with binary files really isn't that hard. If Apple documented the .car format, writing your own parser wouldn't be difficult. It's not like it's a complicated format. Still, Apple clearly doesn't intend for people to make their own .car generators; to them, ease of reverse engineering is a bug, not a feature.