This is not true at all. PNG uses the DEFLATE algorithm which is a perfectly general algorithm for compressing data, it's not just a matter of encoding pixel di

This is not true at all. PNG uses the DEFLATE algorithm which is a perfectly general algorithm for compressing data, it's not just a matter of encoding pixel differences. That's the real reason it doesn't make sense to zip a PNG - the internal data in the image is literally already zipped (the zip format uses DEFLATE). You are however correct that zstd would be a better algorithm than DEFLATE to be the basis of a modern image format - however, that sort of misses the point as well because there are modern techniques for encoding images losslessly that are much better than general data compression can achieve. JPEG-XL has a lossless mode that is "about 35% smaller than PNG" according to their website. That's almost certainly more than you would get out of switching to zstd, or even LZMA.