Why do they call it computer architecture?

Answer 1

Well, computers are fairly complex beasts. Not only do you have to design small, isolated subsystems, but you also have to figure out how your subsystems will combine to form larger subsystems and how the subsystems will combine to form the entire computer.

This is sorta like designing a building, so the analogy is apt.

My dictionary gives one meaning of architecture as 'the complex or carefully designed structure of something'. I think it qualifies.