For example, if you have a Tag Length Value data format that says "5ABCDE" maybe that means "the next 5 bytes are a string, then there's something else afterwards or it's the end", you don't want to allocate a buffer of 5 values and just keep writing into it without checking that you're reading valid values.
Doing this sort of thing efficiently may also mean that you're tempted to remove something like a bounds check. After all, if you already know that the value is 5 bytes, why check on every access? People really want parsers to be fast. Ideally in the above example I wouldn't even need to copy those bytes out, I could just reference those values, which now leads to potential lifetime issues.
Further, in C, buffers don't have lengths attached to them. In every other language you typically don't work with null terminated strings. So now you have to manage sizes of things throughout your parser.
One way to view this is that the programmer's mental model of the parsing machine can easily drift from the implementation of the parsing machine, leading to vulnerabilities.
Basically it ends up being very easy to accidentally end up with out of bounds reads/writes + there's pressure to be fast + formats can be very complex.
That's my view on it at least.