You are comparing potatoes with oranges.
Stories with "adventurers" usually feature "S-rank adventurers" that can level mountains, and more than not, we can see the guild-leader piss at nobles, all the way to the king.
When a commoner can get THAT STRONG, in both authority and in personal might -- it doesn't make sense for the ruling body to allow it.
P.S: I am also pretty sure "being a mercenary" is nothing like "being an adventurer".
A merc afaik, is a sword-hire, not a freelancer that is up to snuff on whatever, and has a convenient merc-ID, with an organization to back them up.
The White Company had a reputation and uniforms (which can work as ID), and they weren't all hired at once. Like people make quests, nobles and villages would hire these mercenary companies to do things that involved armed forces. Although I doubt individual contracts were popular, some free knights would roam around and take jobs alone or in small teams, not belonging to any group. Samurai for hire were also hired to do stuff like guard caravans/merchants.
A merc afaik, is a sword-hire, not a freelancer
I raise you this article talking about the White Company that is literally titled:
While ‘freelancer’ today means independent contract workers, the original term referred to free knights like the mercenaries of the White Company or the ones that went around solo/in small teams.
To clarify tho, I'm not saying The White Company is the same as an Adventurer's Guild, just the closest we had IRL because "killing monsters" and "random odd jobs like cleaning the sewers" were either non-existent or done by people who did them as their profession.
In a world full of monsters, it makes sense for people to make Adventurer's Guilds, and when they do defy nobles/kings, they tend to be international so they can easily threaten to leave their country and let them deal with the monsters themselves.
About the "a Commoner being that strong" part, that's just fantasy, but it's not like the people in charge of the kingdom would know if a random person went to the mountains or go into a crypt and got super strong, and its better for the super strong person to be happy with you than angry at you.
And, in those worlds where 1 person can destroy a mountain, normally there are also monsters that can do the same or worse