Players usually get their quests from very powerful NPCs. If the NPCs weren’t powerful, then players might just be tempted to save themselves some trouble by killing the NPC and taking the reward. Besides, who wants to work for some weakling nobody?
But since quest-dispensing NPCs are powerful, it naturally leads the players to ask them, “If you’re such a badass, why don’t you go do it yourself?”
Good question, really.