Because even the translation “full of grace” is itself imprecise for Luke 1:28, and in the underlying Greek there are two different phrases used to describe Mary and Stephen.
Stephen is “pleres charitos”, which is translated exactly as “full of grace” or “full of favour”.
Mary, however, is “kecharitomene”, which cannot be properly translated into English without a whole lot of awkwardness. “Kecharitomene” is a passive, perfect, vocative participle. The root is “charitos”, grace, favour. Passive means the action of being graced is being done to the subject (Mary). Perfect means that the action took place sometime in the past and effects continue on to the present. Participle means the verb is being used as a modifier, in this case, an adjective, modifying Mary. And finally, its case is vocative, which means it’s being used as a form of address, giving it the function of a title.
Translate that precisely into English and you get, “O you who were graced/favoured before and remain so even today”
“Full of grace” is close but is not precise enough because it loses the continuous sense of the perfect.
To get the full force of this phrase, you have to go to the Greek; not even the Latin captures the power of this address precisely enough, much less any English translation.