It's mostly for historical reasons. More specifically we have restricted shell as text mode only tool, and intellisense-like features/inline help do not tend themselves neatly to line/character based output.

I think that there could be a place for a modern, more graphical orinted command-line interface. In addition to inline help it could display tables more nicely, maybe even allow proportional fonts. And while we are changing stuff, then maybe the object passing idea could be taken from PowerShell.