DND_Reborn
The High Aldwin
As far as fighter levels go:
I know a lot of martial artists - both eastern and western, many of whom are excellent. I know a lot of street fighters who even in their 40's - 70's are getting into fights. I live in Amish country and every weekend they get together and wrestle. Because it seems so common in real life, I can't imagine fighter training not being a natural part of survival in a world where death is often caused by losing a fight.
As far as other classes go:
I live near a spiritualist community and mediumship and healing are not an uncommon pursuits.
I work as a healer and witness miracles quite often, in a professional capacity many of my colleagues do as well.
I know many people who study and practice shamanism.
I know people who have fought bears, survived mass shootings in foreign countries, killed lots of people, won fist fights vs multiple opponents, are expert hunters. Magic IRL is poorly modeled by D&D, but I know many people whose lives are filled with magic. When I start to add up the people in my head who have at least one level and many who I would consider to have 5+ levels It doesn't talk long to be thinking about hundreds of people.
There are "classes" that I don't see many of, but I don't think that they don't exist; they just aren't in my circle of acquaintances. Rogues fall into that category.
While I also know many martial artists and military, most I wouldn't consider to have levels in anything. Just as a "Guard" monster has no levels of Fighter or anything else. Although a Priest can cast spells equal to a 5th-level cleric, he has no levels of Cleric.
IRL, someone who studies religion and practices it doesn't make them a Cleric or Druid. Also, I would like to consider all the people you know who likely wouldn't have any levels in a class. What levels does a cashier have? a car washer? a politician? While you might live in a different type of community, the vast majority of the world is rather mundane IMO. There is nothing wrong with that at all, and many people seem perfectly happy to live mundane lives. Unless a major thing happens to them, they will likely never be adventures.
Either way, thanks for your input.