So what you're saying is that what the character did before the game even started is more important than what they are doing after the game started? What's happening in the game
is the context. Before the game starts, the character is static.
Take a look at mhacdebhandia's players' histories as he's posted them. These are awesome. All of them are acceptable in length, and each of them also contains a bunch of stuff that is useful. On the other hand, getting a three page single spaced typed short story where the character did this, and did that, and did the other thing over there, and then did a whole bunch of other stuff, traveled all over the world, made friends with Elminster, and killed a demon lord, all because his family died (because families always die

) my eyes glaze over and i start wondering "why is this character first level after having done all this stuff, and why is this character even hanging out with the rest of the characters in the first place? How did a first level character kill a demon lord?" If it's filled with stuff like "Rogar the Demonslayer has blue eyes and likes bananas", well, frankly, i don't care what color your character's eyes are and what his favorite food is. That's up to the player to roleplay.
A long character history is generally indicative that the player wants their character to do a whole bunch of cool stuff, and so has their character do all that cool stuff before the game even starts, possibly because they're afraid their character will never get to do anything cool again, or possibly because they just enjoy writing. Who knows? There could be any number of reasons for a person wanting to spend that amount of time writing up a long, involved backstory that usually excludes the other PCs. At any rate, all the cool stuff has been done before the game has started, and i'm left asking "Well, if you did all this cool stuff, why is your character willing to go into sewers and whack rats with these other guys?"
I counter your comment of my anecdotal experience with my own comment of your anecdotal experience.

The vast majority of people who play are not published authors. Also, source, or it never happened.