Because these extra chunk are super-obvious? As soon as someone looks in your file they're going to wonder what you've added these for.

They will likely also not survive re-encoding.

The point of modern stenography is that you can't tell it's there, and it's just a part of the core image data, and that it's completely deniable.

How do you deny your nonstandard chunks?

Otherwise, you dismiss just about any examples of steganography prior to steganography on computers.

Which is fine, but as far as i know, not the commonly accepted definition.

> so why bother

The answer is - because it's more deniable. You bother because more deniable is better.