Here is a great answer to this question from the NASA website:
“People have considered harnessing lightning for electrical power, but no serious attempts are currently under way, nor are any planned. It is impractical for numerous reasons, as listed below.
1. Most of the power in lightning is dissipated as thunder and light, which cannot be easily harnessed to generate electric power.
2. However, even just the electrical current of lightning is considerable — 20,000 amps on average, the same as 100 steel welders. But the power is on for only a brief fraction of a second, so the total power is actually small, only enough to power a 100-watt light bulb for six months.
3. The huge surge of electrical current over very brief times makes storing the energy impractical.
4. Lightning strikes to specific locations are infrequent and inconsistent. This makes scheduling power availability impractical.
5. Lightning can be very damaging. The collection systems have to be incredibly robust, which would drive up costs. Lightning is also very dangerous, making it tough on the collection system workers.
6. Lightning would have to be collected over a huge area, making the system impractically expensive.”
Well there you have it from Jack. Put simply – too much energy for too short a period of time. I actually possess the remains of a lightening strike. It is a long piece of sand fused into a glass by the immense heat of a lightning strike released when it hit the ground.
If you are caught in a thunderstorm, do not shelter under trees (tall objects act as lightening conductors) and do not stand all alone in the open if that makes you the tallest thing around (because then you become the lightening conductor). If you can, get inside a car. The metal of the car acts like a “Faraday cage” spreading any lightening strike around and away from you.