Why does it matter if it =HAD= checksum, the numbers would have been altered prior to save. It means you store one but you get two when read later. If the format calculates immediate checksums on blocks it'd detect memory corruption at best. The extreme downside is that such a part is untestable under normal conditions, hard to maintain, and it costs more than the ECC in development.

That happens more than you think though. Most* things that output PNG are making an in-memory data structure and dumping it to disk.