The structural steel of a building does conduct lightning, but you need a lightning protection system to provide the necessary interconnections to make sure that the lightning is able to pass harmlessly through the building.
Without the lightning protection system, when a building is struck the lightning attempts to find a path to ground. If there are not enough interconnections providing a continuous path for the lightning to follow, there will be arcing or side-flashing. The lightning will also travel on other mechanical systems in a building such as the electrical or HVAC systems. The lightning current jumping from object to object within a building is dangerous and has been known to cause fires, explosions, etc.