For the same reason why there is a OSI-reference protocol stack which is (almost) nowhere implemented. It is an ideal against which each "real existing implementation" is measured. Even if no implementation fully conforms to it every one is - at least in theory - striving to do so and asymptotically converging towards it.
Take the OSI-reference model for networks as an example: to my knowledge the whole seven layers of it where only once being implemented (in some obscure DEC network stack working under VMS), but every network protocol is compared against it and we say that "IP is a layer-3 protocol", even though TCP/IP was not built with the OSI reference model in mind and in fact - from a theoretical POV - really covers only the layers 3, 4 and 5.
I hope this helps.
bakunin