In case of epic quest to save the world, it's never the Royal Guards or the veteran that we send. Because the guard must protect the Royal Family and the veteran are either too old or already dead. So it's always the rookies adventurers who will be blocked by a locked wooden door. -- Anon Guest
They had tried, once, and only once, to send the Great Hero to defeat the evil forces of darkness. What they discovered was the Unwritten Rule: Whatever you set out to face down will be the greatest challenge you have ever encountered. To send a Great Hero after the big bad was to invite horrors against which none were truly prepared. Not even the Great Hero.
It's a fable, now. A fairytale. But the Adventurers know the truth of it. Newbies just starting out don't face foes more powerful than Goblins and the occasional Orc. When they get up there, they start facing down Eldritch Horrors and otherworldly entities set on eating existence as it is known.
Rulers and nobles certainly know this, and publish a great reward (that will be inevitably claimed by a band of kids in gimcrack armour and some newly-acquired cool things) for ridding the realm of some slightly-nasty foe as a form of test before going against the true threat.