Because the average person wouldn't want a 300W computer + 100W monitor running all the time. Besides, if you're still on a modem, you probably want your telephone to not be occupied all the time.
It keeps the bills down and let's face it: There's no usecase for an average person to have a computer running all the time. It's like having your TV and radio running all the time. You could do it, but you better shouldn't for various reasons.
Monitors have power buttons and desktops have little reason to use anywhere even close to 300W when idle - or do you leave 3D games running when you're computer is not in use? Anyway, since we're talking about your grandfather here, his computer really doesn't need to be all that powerful, to theres plenty of opportunities to keep power usage way way down.
Having said that, I completely understand your argument and agree that an iPad which is always connected on a 3g data plan, can be carried around and beeps when messages arrive really is easier and more old-person friendly.
It keeps the bills down and let's face it: There's no usecase for an average person to have a computer running all the time. It's like having your TV and radio running all the time. You could do it, but you better shouldn't for various reasons.