Why not have both options? .gltf and .glb being possible for assets been more than helpful to me more than once, having the option gives you the best of both worlds :)
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.
Computers need to do it a bunch for every program launch for every single user of macOS for decades. The developer just needed to write a generator and a parser for the format once.
Would it have been a bit easier to write a parser for a format that's based around a zip file with a manifest.json? I don't know, maybe. You'd end up with some pretty huge dependencies (a zip file reader, a JSON parser), but maybe it'd be slightly easier. Is it worth it?