Given how ubiquitous the concept has become in Japanese media, there must be people who sincerely believe that adventurers' guilds actually existed in medieval Europe.
Funny you should say that.... They sort of did... many of the occupational Guilds used a system
similar to what you'd see in a "standard" fantasy adverturer's guild to divide the available work.
No Monster Extermination Requests, but you
did have mercenary brokers.
Hanzebund Chapters also had an "odd job" desk where requests could be made, which were then posted and taken up by whoever wanted/needed the job.
Not
quite the same, but the general gist of requests being made to a guild, who then makes them available to their members, with the better requests being available to members with a proven track record/higher status did exist.
And was actually an important part of the economic engine(s) of the time, especially for the cities.
So not a matter of belief, but historical fact.