Macs are built as appliances. You don't see people asking at the electronics store "Hey, I bought this fridge the other day but now I want a 2000BTU compressor, can you get someone to upgrade it for me?"
Apart from the Mac Pro no Mac is upgradeable, apart from little concessions like ram and HD. Oh and by the way that is probably because Apple is a small player trying to win fans now. Back in the day Macs do have ram chips soldered right into their slots.