Some formats are simple and the fields don't interact with each other at all, some are complex and the format changes depending on other values.

Parsing is hard because you have to handle all the possible inputs someone could throw at you, and depending on the format that can leave hundreds of very rare edge case no reasonable human would normally think of.

This is also why fuzzing is so effective on parser, fuzzers are great at throwing many different combinations at the wall until they find a new interesting edge case, and jumping off from there to see if they can mutate it into more.