Because when compared to other means terrorism is an efficient and effective means of achieving a goal, especially in warfare, terror has managed to exist over the years. The primary achievements of terrorism have been realized throughout history when weaker military means have given way to terrorist methods.
Terrorism fights wars in an anonymous identification from anonymous locations and is therefore clandestine in nature. Although names identify terrorists their identity and location are not known in most instances. This is one of the first advantages of being a terrorist combatant or saboteur. This results in less retribution from terrorist targets upon terror organizations.
Terrorism also prevails by presenting the opportunities to organize, equip and train their terrorists for just a pittance compared to the war budgets of most nations that support military organizations for protection and counter attacks. In fact, most terrorist organizations use other nation's money and equipment to wage terror. Some even grow into a kind of business by aligning with supporting nations in secret. Potential terrorist personnel are ever abundant in the poverty stricken world and especially in the nations promoting religious zealotry. Therefore the upper end of the food chain among terrorists usually occur from religious zealots with their own means and they approach a more capitalistic nature as they are usually closer to the financial and power business goals. This may be evidenced by the fact that fewer of them are available for personal suicide missions. These facts contribute to the success and longevity of terrorist organizations. They also provide some breadth for the organizational spectra of terrorist organizations.
Terror training has been accelerated by the ease of modern weapons, the simplicity of weapons of convenience such as commercial aircraft, fuel trucks, rail and sub ways and bio weapons. For the simple explosives it is comparable to training rats to negotiate a maze in the least time to receive the reward of cheese or religious acceptance and multiple virgins. For the unwilling there is easy inducement by the threat of killing of their family or just fear of their own death as an alternative. Thus terrorism has another advantage in the waging of war.
While satellite surveillance finds preventive measures are necessary for an enemy nation's development of warring materials or WMD proliferation, the terrorist may operate as the salt added to the flour while preparations are made to attack from within a target nation's territory and accumulation of the weapons required with impunity as the satellite surveillance sees only dough of the baker in the bakery. Add to this fact the abundance of targets in a target rich country and you have considerable advantages when you're on the terrorist menu.
Political considerations are an advantage when selecting terrorism as a means of war. There is little need for elections of taking responsibility when terrorist success is not realized to the expected extent the terrorist's desire. Another factor is the fact which works politically for the terrorists is the ability to change allies or sympathizers in mid stream without repercussions. Coupled with the fact that many in the world sympathize with the terrorists and considered them the David fighting Goliath, the Zorro's or the Robinhoods against the evil King world bodies are of little help in opposing the terrorists.
Terrorism may even have an advantage in countries like Iraq when leaders like Saddam Hussein are in control. In this case a cruel ruler cements his position and lessens opposition through terror control of the citizens of Iraq. In these instances the terror is continuously applied and is less costly for Saddam.
It is easily recognized that terrorists have many advantages in war and with ever increasing success in wars they will be present in more abundance in the future. In summation it may be said that terrorism exists because of the past success of terrorist movements.