Why?
Those are completely different devices for completely different tasks. How you use/interact with a desktop machine doesn't inform how you use/interact with a handheld mobile device such as a phone (something which Microsoft has yet to figure out).
You're right about "completely different devices for completely different tasks" but that has nothing to do with a File Manager. File managing isn't a task - it's part of organizing.
People with iPads (who use iWork or Garageband/iMovie) are doing tasks that still require organizing. Organizing by App isn't ideal when dealing with different file types.
Personally, on my iPad, I would like to have all my documents organized by year & then by classes (ex. Econ, Pop Culture, Film Studies). However, those documents are mixed media (ex. RTF file, Pages Document, PDF) and I cannot do that. It is frustrating.