/prog/ challenge - Compressing Visual Novels

What makes VN companies come up with proprietary image/video/audio formats that compress worse than standard ones? Here's an example:

Proprietary format, original: 871KB
Proprietary format, recompressed[1]: 822KB
PNG: 676KB
Optimised PNG: 674KB
JPG (high): 221KB
JPG (medium): 125KB
JPG (low, artifacts start becoming obvious): 86.7KB

So they could be several times smaller while looking almost the same, and that's not even getting into things like difference encoding since the bulk of them are almost the same images with slight differences.

Here's the challenge: find an existing VN, and write a little utility to compress it to 1/2th its original size or less while maintaining all the content. You have until the end of the year.

[1] I RE'd the format and wrote a compressor for it, which performed better than the original while decompressing to the exact same data.

Just the other day there was a libJPEG arbitrary code execution exploit released and here you are asking why no sensible developer wants to use open standard trash. A better thing to do would be just use DirectX texture files for everything, since it's developed and maintained by real programmers good enough to be paid, not college dropout failures who work for exposure and are terrified of showers.