Someone Always Hates the King

No land is monolithic. No political party is all-encompassing. Nowhere is perfectly, 100% stable. In other words: someone always hates the king. There are a thousand reasons that someone might hate the king. To name just a few: he's unjust, he promulgated a law which injured someone's standing (or pride), he's increased the taxes, he's decreased the taxes, etc. ad infinitum.

This means there is always maneuvering going on. No kingdom is ever static and safe, which is great news for you. Whether it's a powerful underlord of crime, dangerous nobility, foreign interests (usually coupled with one of the previous two), unstable mercenaries, outlaws and rebels, or whatever you can think of, someone always hates the king.

No rule is secure, which means adventurers always have work to do. Putting down a dissenting group will just result in creating another, people who now despise the adventurers for their meddling. This is because adventuring is always a political act; it is, above all things, an expression of power. He who has the power to hire adventurers has the power to challenge (or reinforce) the state. Adventurers are a third wheel, an extra gear, a way to solve problems without using normal channels. After all, you can always deny you hired them.

If a nobleman sends his knights to attack the king's procession, someone is bound to recognize them. Even if they're masked and hooded, the word will get out that he raised them from their farms, armed them, and sent them out. Besides which, just let one be captured and the whole house of cards (tiles, in Arunia :V) will come crashing down around their ears.

However, if that same nobleman hires adventurers in secret, demands their discretion as to their paymaster, no one would ever be the wiser unless they were caught and possibly tortured. Of course, the problem with adventurers is that they are generally essentially mercenary, so whoever has the deepest purse tends to control them.

Political instability is always good for adventuring, though it may be bad for individual adventurers (particularly if they are themselves devoted to one side of the cause or the other). The fact that someone always hates the king is good for business, because you can always get work with the people who hate him or with the king himself, making sure those people don't topple his kingdom.