But using JSON would have required parsing text in order to load the file. That's fine in a lot of domains, but games often have to load megs and megs of data in a very short time.

To speed things up, lots of games have this sort of external program that reads in a human-readable text format and outputs cooked binary files (with appropriate endianness and so forth) for the game runtime to consume. The fact that this utility is an external program means that there's not a reason not to use a nice language instead of a C library.

For a toy example like this, the advantage isn't obvious, but when you're consuming dozens or hundreds of megs of compiled text, it's indispensable.