I understand where you're coming from - overcoming a challenge is rewarding. That's why people challenge themselves.

But some players are not always interested in a challenge, or maybe less of one than is being presented. Perhaps they really enjoy the quest line, but this one monster makes it unpleasant for them to play. There are as many reasons as there are players, and while you may find it silly that some choose to quit rather than play a game they find more challenging than desired, it's a valid opinion for those people. And we don't want to tell them to go play someplace else - they deserve to play the way they want to.
We can't always please everyone, but we can please as many people as possible. By selling an item in the store rather than changing the game as a whole, we allow people to customize their experience in a way that does not directly affect other players.