APNG's palette mode is much worse than GIF, and RGBA mode has 3-4 times more data to compress. Deflate is better than LZW, but not that much better, so APNG can only offer either worse quality or worse file size.

Lots of large GIFs used today were converted from H.264 (YouTube, etc.), so the crap quality of H.264 -> GIF -> H.264 is entirely the fault of GIF.