Here are some thoughts, take them for what you will:
I suspect it has something to do with attitude towards what "typical" people are. In a campaign where there are lots of high-level NPCs floating around (three archmages in every city, and the commander of the City Watch is a 14th level fighter) it wouldn't be a big deal to just handwave it and say "your new 14th level fighter is a recently-retired commander of the City Watch who now wants something more exciting than a desk job." However, in a campaign like mine where there basically aren't any high-level characters except the PCs and maybe a few drow matriachs or hibernating liches, it's a lot harder and less satisfying to explain your new PC: "I guess he killed a lot of criminals over the years? Or maybe there was some kind of a quest, that we never heard about, where he saved the city from vampires or something?"
Here's the thing: players are Weirdness Magnets. If there's a murder in the city, the PCs have a statistically much-better-than-average chance of being witnesses to the murder, or suspects, or victims, or something. It's implicit in the social contract for the game that the DM will set up interesting things to happen to you over the course of the campaign. As soon as a player becomes attached to a character, bam! the PC is now all set to rise out of the ranks of mundanity, see mind-blowing spectacles, go to new lands, meet new and interesting people, and then kill them (as the old Army joke goes). If you play from level 1, there is no question where that level 14 fighter came from: he was rescued from goblins by a band of hobbits, organized the hobbits into a fighting force and retook the hobbit homeland from drow, stole a magic sword from the dragon Ferrovax's hoard, bluffed the Fire Giant King into recognizing the hobbit kingdom, and fought down a vampire cult that was kidnapping and vampirizing hobbit maidens. Of course, you could try to write a backstory that said the exact same thing, but without it having happened in the spotlight during game time, 1.) it is less memorable, detailed, and interesting; and 2.) nobody cares about your character except you. Often not even the DM. By letting his "backstory" happen on stage, with the other PCs, they become equally invested in that story, and it's a shared experience and not just a background monologue.
TLDR; human psychology dictates that remarkable backstories aren't allowed to happen offstage. Ergo, high-level starting characters are effectively required to have unremarkable backstories, which rubs some people the wrong way.