Focal length is directly connected with the perspective of the scene and the view-angle. (One usually doesn't switch bodies to alter the viewfield so I humbly ignore the effect of the sensor dimensions.) Therefore you have the information already in the viewfinder, but not as a single value. If there was a dedicated display for such value it would occupy room for the other values to be displayed. One would have to decide whether to reduce the size of the displays (reducing readability), reduce the viewfinder (reduce the viewfinder accuracy) or remove some other display.
Values for aperture, EV, shutter, and ISO are displayed as numbers (bar scale) because the visualisation may introduce more issues than benefits.
The aperture is usually wide open and closes to the selected setup only when the shutter is released. Reasoning is simple:
- Preview is brighter than the actual image alowing more precise focus
- Wide open lens have shallower depth of focus alowing more precise focus
If you want to see the actual DOF, you can temporarily close the aperture by the button near the mount (depends on camera design).
Effect of shutter and ISO setup is impossible to preview and EV is calculated from measured light intensity and the values.
If you want to see the actual value of the focal length look at the lens from above. The "zoom" ring has several values printed (engraved) on it. Here you can see a Canon 70-200 mm f/2.8L lens where focal length is set to 200 mm and focused on object 1.5 m away from the camera.
If you are asked to shoot at FL = 90 mm, set the ring accordingly; then look in the viewfinder and adjust the scene by walking around to get the desired scene.