You then learn from you experience and you become a better player. You become more observant of what your opponent is doing and where you are. You use different characters and experiment with different skills to see what is good. Since there are five different classes and 150 skills, there are a lot of possibilities. Most of the characters follow a certain trend though and people find ways to defeat that kind of character.
Some people go so far as to make characters specifically made to defeat a certain kind of character.
To combat this, people used their intelligence to create characters specifically made to destroy them and did so very well. Some players go so far as to create characters to be able to kill other player character builds. These people would post on forums about their character and defeat a large number of people easily and then someone else would create a character that would beat that character easily. So my second reason about why people enjoy games so much is that they enjoy the challenge of them. One of the most important aspects of most games including rpgs is the difficulty factor.