The answer is because they want you to keep playing.
I know it's kind of cynical, but think about it from the perspective of the developers. Do you really think their only goal is to provide fun content? They aren't saints or martyrs. They have a bottom line to meet, and in order to keep making money, they have to keep people in the game as often and for as long as possible.
Now there are various ways to reach this goal. One is to keep people engaged through interesting content. The other way is to force farming of gear every season/tier.