I think they do that to make up for the fact that most main characters in WRPG's are user created. They need a hook, and making you the chosen one makes up for having no consistent personality.
This just exposes how lazy they are in their writing. You can still create a history about a character that is user created and still not making them a special snowflake.
How so?
Thing is, people HATE the intro to FO3 despite it having very good storytelling.

In Suikoden games, you have a history, a family, a set of friends, an identity and none of the main characters even TALK (except in III). Surely that's not below wrpgs to be able to pull off.
This is what I loved about Dragon Age: Origins. You had actual origins and the game shows you and let's you experience them.
Most wrpgs are like, pick a line about what your history is (Vamp: TM, Mass Effect) or create your own (D&D style).
They're typically not about establishing history due to the very creation of the genre and its influences (pen and paper). It's not a necessarily bad thing, but it's something that sticks out that I don't like. At least in video games. In D&D, I love it.