A little over a decade ago, I began what I call a character roster. (I never play myself, far too much suspension of disbelief.) They have names, personality traits, fighting styles, etc. For various games that allow a name entered, whether Role-Playing or otherwise, I use one of them as appropriate. Obviously in some games it's easier to use some than others, and occasionally I've had to add one to fit into the newest role.
However another trick is to take one that maybe doesn't exactly fit, and take the good ol' comic-book route and say "...but what if?" An alternate-universe version. And that's my plan for Fallout 4. One of my female characters, though never married or with child, might well have been if circumstances had been ever so slightly different. Otherwise she's a good fit already, ex-military, probably a sniper in this game but who knows. Might even go Amoral Assassin with her again this time, 'twas fun in a previous game.
So no, I honestly don't get the complaints about having a more-concrete backstory. Headcanon characters can evolve and adapt if you're creative enough, and if you're incapable or unwilling, well, where did you get the idea for the first one? What's wrong with creating a new one? Otherwise it seems to me you merely want to play the same game over and over again.