Objects aren't human readable and if they get corrupted the object's format is very difficult to recover, you need an intimate knowledge of both the specific format used (of which there might be thousands of variations). Plaintext, however, if that gets corrupted it's human-readable. The data might not be more compact (Although TSV files tend to come out equal), but it's more robust against those kinds of changes.
Have you ever examined a protobuf file without knowing what the protobuf structure is? I have, as part of various reverse-engineering I did. It's a nightmare, and without documentation (That's frequently out of date or nonexistent) it's almost impossible to figure out what the data actually is, and you never know if you've got it right. Even having part of the recovered data, I can't figure out what the rest of it is.
