Because I need to:
1) Touch the keys and feel them being pressed.
2) See the screen without it being obscured.
3) Be able to type on something sensibly laid out and appropriately spaced with a proper layout and not have to switch between different bloody modes to type numbers or symbols.
They're impossible to type as quickly on, they block what I'm trying to see, and they're crap.
(And don't even think of mentioning that silly Swype shit!)