Why are depth buffer values clamped to (0, 1)?
(Just converting comment into an answer)
It seems you might be asking one of two possible questions.

If you meant "Why doesn't the Depth buffer contain camera space Z"
then it's perhaps worth reading Why do GPUs divide clip space Z by W, for position? 

If, OTOH, you just meant "why is it restricted to [0,1)?", then that's probably because old systems represented the Z-buffer depth with fixed point numbers, hence a natural [0,1) range.
 Of course, later hardware arrived that could use floating-point depths; IIRC Dreamcast's depth representation was an arbitrary positive float representing $\frac{1}{w}$.