"Roll" is the name for one of the three axes in aviation. "Vertical" and "horizontal" aren't very descriptive in three dimensions.
You could reasonably call yaw the "horizontal axis", but then assigning "vertical" to either of roll or pitch – and what do you call the other one then? They're arguably both vertical, depending on which side you look at the plane from! Additionally, at least "horizontal" implies an Earth-centered focus, which doesn't help while in, say, a barrel roll :)
Best to avoid the ambiguity entirely and use specific terms, just like how port and starboard avoid the "my left/right or yours" ambiguity nicely by always referring to the ship's frame of reference.
You could reasonably call yaw the "horizontal axis", but then assigning "vertical" to either of roll or pitch – and what do you call the other one then? They're arguably both vertical, depending on which side you look at the plane from! Additionally, at least "horizontal" implies an Earth-centered focus, which doesn't help while in, say, a barrel roll :)
Best to avoid the ambiguity entirely and use specific terms, just like how port and starboard avoid the "my left/right or yours" ambiguity nicely by always referring to the ship's frame of reference.