While lighting does have large amounts of energy, the lack of predictability severly limits how we can "collect" this energy and use it. If we look at other sources of energy, such as burning fossil fuels, we can predict when and where the energy will be generated. We can also generate the energy as its needed (assuming we have sufficient amounts of the fuel). The same goes for other forms of energy such as solar and wind energy. While there may be variations in wind speed or cloud levels affecting solar energy, we see an average over an extended period of time with reasonable fluctations. For lightning, we cannot predict where it will occur and there are significant variations in the amount of lightning at different times of the year.
we cant such harness it because we cant be certain where the lightning would strike , it has to be some mobile truck with a large static drum at its back but this method of producing electricity is not very sustainable