"There's more people who want to level than there are real fights to level them.  So the governments control the dungeons, and the amount of taxes and qualification tests and Adventurer's Guild bullshit and random quests that city lords throw at you and all of the other pain and inconvenience rises to a level where the demand to be an Adventurer is low enough to meet the number of dungeons.  If there was excess demand and more people wanting to be Adventurers than there are dungeons, the Guild and the governments would have room to make things worse again, so they would, because that's how people are if nothing is stopping them."

"And, I mean, sure, there's normal monsters too, but nobody'd hire anything but an appropriately ranked Guild team because anybody else you send is just going to die, right, and if they don't you'll get in trouble with the Guild about it."

"There's more people who want to level than there are real fights to level them.  So the governments control the dungeons, and the amount of taxes and qualification tests and Adventurer's Guild bullshit and random quests that city lords throw at you and mandatory service with long hours, plus all of the other pain and inconvenience, rises to a level where the demand to be an Adventurer is low enough to meet the number of dungeons.  If there was excess demand and more people wanting to be Adventurers than there are dungeons, the Guild and the governments would have room to make things worse again, so they would, because that's how people are if nothing is stopping them."

"And, I mean, sure, there's normal monsters too, but nobody'd hire anything but an appropriately ranked Guild team because anybody else you send is just going to die, right, and if they don't you'll get in trouble with the Guild about it."