Maxperson
Morkus from Orkus
It's not that PCs are the only examples of wizards, fighters, etc. It's that the vast majority of other wizards get better by sitting in those guilds and practicing. And the fighters by sparring with one another and practicing, with maybe the occasional bandit hunt. It's slower going and they tend to end up lower in level at the end of the day, but they aren't out there fighting hordes of monsters that are threatening all the cities.I'm not particularly interested in PCs being particularly exceptional actors in the world. Or, if they are, then I generally want them to belong to a class of exceptional people that is larger than my 4 players, and I need to figure out what impact that has on the broader setting. What is exceptional about them, how do other people using PC progression affect the world, and so on. If they're quite rare, then adventurers are probably a national resource and a lot of effort is going to be put into cultivating them.
Wizards, for example, are conceptually constrained if it's not a broadly available skillset. You can't really do magical academies, but you could focus on a more archaeological tradition of digging up ancient scrolls, or perhaps a lone scientist doing practical experiments to learn new spells. That kind of exploration about what any given mechanic means to the broader setting is you know, half of what I'm here for.
The PCs are the ones encountering extraordinary numbers of monsters, because that's what makes a fun game.