I don't recall us ever finding out what happened to the mum, but the fact that no one ever talks about where she is (plus Carly and Spencer never contact her) suggests that maybe she did pass away.
And yes, we do know that their dad is working overseas. I suppose there's a chance he might be in an episode one day (if he comes home to visit) but his job in the navy is why he has not been seen on screen yet.
They don't appear on the screen because it's irrelevant for the plot. In Dan Schneider's other show, Victorious, Trina's and Tori's parents hardly appear, and if they do it's not rellevant for the acutal happening.
iCarly is about three teenagers who host their own succesful webshow, it wouldn't have made a change if Carly's and Spencer's parents appeared.