iPad is made from milled and polished Aluminum and a fine polished glass surface. That is 3 to 5 times what a damage proof plastic enclosure costs.
I'm sure the iPad1 was simply, quickly and cheaply just stamped out of a sheet of metal, not machined out like the Macs are.
You cal tell that by the teardowns that show internal fixing points stuck onto the surface of the metal inside.
Also you'd never have a curved back as the iPad1 and machine it out. I'm sure it's just cheaply stamped.
I'm not saying there's anything wrong with that. Just it needs to be realised.
Looking at the iPad2 teardown, the current iPad2 model does look machined out.