Because its just not true.
https ://www.wowhead.com/wotlk/spell=48265/unholy-presence
There is Unholy Presence reducing the GCD by .5sec in Wrath.
https ://www.wowhead.com/cata/spell=48265/unholy-presence
There it is for cata, reducing the GCD by .5sec.
The baseline GCD was 1.5 reduced by .5sec in both Wrath and Cata.
Also, during this time you couldnt reduce your GCD under 1 second so it would be kind of pointless to have a 1 sec GCD and a .5sec reduction when it couldnt be reduced under 1 second.