Keep all the meta info in JSON and then the big binary files in a zip file. Much easier to parse.

Easier for the developer or easier for the computer?

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?