This is a good question. I think every martial artist with an analytical mind has probably wondered the same thing.
The answer is simple—it is because Taekwondo a martial ART.
Warriors love to fight. When there is peace and no enemy around to fight, they found that, if they were careful, they could fight each other. This sitting around and training during peacetime led to the development of fighting systems. During times of war, you are using techniques, keeping what works, and rejecting what does not work, but you do not have time to think about putting it all together into a system.
Due to having to camouflage the meaning of their movements, or while to tying to find a way to safely train, they began using patterns as way to practice. Gradually, the practice of the patterns developed into an art form, almost a separate entity from the fighting system it represented. The fighting (martial) systems became known as arts, and then became know as martial arts.