Why is it....

That fixed gears use threads/lock rings to being with?

Coaster brake bikes, well some, use C-Clips, and I could imagine that being a more efficient method, since the cog isn't supposed to move anyway.

Or why not use a system like cranks? A square taper on the drive side of the axel that the cog fits on to?

I mean I am sure there are reasons, but why not?

Is there a superiority to the threaded systems? Lighter maybe? Simpler? (can't imagine something simpler than a C-clip though)