The PC probably needs to come up with a background that fits within the framework of the game being played. The same criticism could be thrown at any of the other classes. What if the PC background is that he was a humble baker before being caught up in adventuring, and not a Fighter? Does he automatically start out underpowered because of his background or is he suddenly skilled in combat and can wear heavy armor with ease?
Samwise Gamgee was a humble gardener.
The idea of heroes (player characters) coming from humble origins is hardly anything new or revolutionary. Joseph Campbell wrote very famous books about the entire concept.
The idea of playing a novice priest in a local temple, who sees his hometown burned by orcs and joins up with friends to avenge their lost homes is not "within the framework of the game being played?" because the PC cleric was a local priest and not a trained militant Cleric? That sounds like a classic D&D Player Character backstory to me.
I remember the very first D&D product I ever bought, the 1991 Basic D&D "Black Box" set with Zanzer Tem's Dungeon as the module. That 20+ year old introductory module starts out with characters randomly rolling for their backgrounds before the adventure (basically being randomly thrown into the town dungeon for petty offenses), with pre-adventure backgrounds including apprentice baker and scullery maid.
In the ~15 years I have been gaming, players that bother to give their PC's a detailed origin far more often than not give them a humble background than one of extensive training and preparation. IME, only Wizards, Monks and Paladins typically have extensive training.
So, it's a standard of fantasy fiction, and D&D, that PCs often come from humble origins and haven't spent years of their life training for adventuring before the first time they set foot in a dungeon (or other adventuring setting). Of course, most PC classes are front-loaded with abilities that presume they have been.
The only exception I can think of is the Wizard. The whole reason the Wizard class is so limited in weapons and HP is the presumption they have been bookish scholars their entire lives instead of training to fight.
It's a logical paradox of the game. PC's from humble origins but with class abilities at 1st level that presume extensive training.