Because Blizzard is adamant that the melee Hunter spec is definitely Survival, and not Beast Master.
Rexxar has been a Survival hunter all the way back to Legion when the melee switch happened and I guess someone at Blizzard realized the error of having someone called a master of beasts melee was just confusing, so they made him Survival and try their hardest never to mention his beast master title.
They even had a survivalist themed questline for Rexxar out of nowhere, seemingly just to really hammer in the Survival theme.
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Real shame that Survival cannot dual wield as well. They only have to use a single two-handed weapon because a single one of their abilities is hard coded to only work with that kind of weapon. Remove that restriction and there is nothing preventing Survival from using two agility weapons instead.