As far as tea and coffee are concerned, it's because most tea and coffee is blended from true varieties. In the case of tea, it's Assam, Darjeeling and others. Coffee can be a blend of Colombian, Ethiopian and a whole lot of others. All these different varieties contain different amounts of caffeine. On top of this, your average spoonful of Nescafe and Gold Blend etc is subject to the whims of the Nestle blender, which means that the caffeine level will differ between batches. This means the true caffeine level can never be calculated with any accuracy.
The FSA and European authorities accept this difficulty and have never insisted on disclosure of caffeine levels in blended coffee. If some individual manufacturers want to try to provide a figure, they won't stop them.
Caffeine disclosure for soft drinks is not necessary under current regulations with the exception that Buenchico has mentioned above.