In order for a background story to make sense it needs to be integrated into the conversations throughout the game. Having too many background stories to choose from would likely make things too complex for the designers and most likely to fail. But having no background story at all feels a bit poor. On the other hand, having one fixed background story could make it hard to justify certain races and classes, unless very elegantly written.
Before we complain too much about this, maybe we should wait for the full release. There are some hints that BG3 will link to the story in BG2 in ways we have yet to discover. Perhaps we have just yet to experience the background story of the main character (and companions), idk.
