The question should be,
Why isn't there a way to rotate legend components?
Legends are great, for one reason--you can place a legend on more than one sheet! And that is exactly what you need to be able to do with a legend. However, when I began to use legends, I quickly discovered that dragging a family into a legend view didn't work very well. A few families behave "properly"--that is, you can select the view you want for the legend. Doors work well. But, if you are an MEP designer, you are out of luck in most cases. If you select the front view of a panelboard, for example, it might be the front view, or it might be a side view. Or, you might get a front view with the panelboard lying on its side. If you are lucky, you might just have to select a back view to get a front view.
One of Revit's strengths is that if you change something on one view of the model, it changes in every view, because views are just that--views of a single model. However, it a real design project, you need to have "views" that are not part of the model. For example, an HVAC designer needs to have a schematic view of the HVAC system to show logical relationships that cannot be conveyed in the basic 3D model. A set of electrical drawings needs to include a riser diagram, or a single-line diagram of the electrial systems. It would be very useful to be able to use legend components in these schematic views--you could use a view of a pump, the same pump used in the model, as a symbol in the HVAC schematic diagram, or an elevation view of a panelboard in a riser diagram, instead of searching for suitable symbols, or pulling in a 2D AutoCAD drawing, or any of the other "workarounds" you can find in the various forums.
If you have to use a workaround to accomplish a task, then something is missing from your basic toolbox! You can layout a right angle with a knotted string, but a square is much handier.
Revit needs an "auxiliary" design space. Yes, you can use phases or design options to address some of the tasks mentioned above. However, legends would fill the bill, if they just worked!