Why couldn't MAC addresses be used instead of IPv4|6 for networking?

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http://serverfault.com – I am reading up on TCP/IP and other related protocols and technologies. MAC addresses are described as being (reasonably :) unique, and as having a large possibility space (several hundred trillions), while also being assigned to all network interfaces. What are the historical and technical reasons why IPv4 or IPv6 addresses are used instead of MAC addresses for internetwork communication? Am I missing something fundamental or is it just a silly reason (e.g. building on top of legacy tech)? (HowTos)