Because that's how NPCs roll in 3/3.5/PF. Why would the book have 20th level commoners in it if they weren't a thing?but why do we need to be 3rd or 8th level? why not just a master chief?
"Holy cats! Did you see that lone peasant (52hp, BAB +10/+5) take on that young knight? How did they become that skilled?"
"They took their farming very seriously and are the best at it in the world! You should see the retired master Blacksmith over in the next kingdom!"