technically apple computer is a set of transistors, a little bit of plastic and aluminium, but this is not important. Concept of value comes from user's perception. If something is seen as valued $1, then it becomes $1.
How you call your dollars in bank account? "Bits, ones and zeros which represents my money" or just "my dollars"?
A computer is a combination of many vastly different things whereas a SBD is just a collection of steem. Calling a collection of steem "dollars" is erroneous and misleading to those who don't understand what they really are.