Fluff and class features.And there are many who don't. It's just something happens and "bam" you are a superhero. You can have a highly trained 1st level character, but there is no requirement that they be highly trained.
The Farmhand goes from:
1 HD commoner (2 HP, proficiency with club)
to
2 HD guard or warrior (11 HP, proficiency with light armor, medium armor, shields, all weapons)
to
level 1 Fighter (12 HP, proficiency with all armors, all weapons, shields, 2 saves, 1 Fighting style, Second Wind)