That is an opinion, not a fact. I didn't give one **** about Shepard's back story, because it wasn't one I would've written. It didn't enhance my gaming experience, it hampered it because I felt shoehorned into a specific person, whereas leaving it more open allows the player to interpret the character as they'd like to.
It's not that hard, it's called imagination.
Sounds like you should try out a Bethesda game instead. The Elder Scrolls and Fallout 3 + NV have a looot of room for the kind of imagination you seem to miss. I'd rather Bioware kept to their own way of story-telling, however.
A way I thought of doing this would be to write a bunch of characters and let the player pick which one to play while using the others as squadmates. Each one would have a surname and a default class, personality, appearance, and first name. Although there would be more dialogue to write, there would be no additional voice actors needed and from a storytelling replay perspective you could learn about a characters back story as a companion and then delve into more depth by playing as them. The writers wouldn't have to write any more back stories at all, just flesh them out and change opening depending on player choice.
Now that's actually really interesting! Kudos for coming up with something original
it would be quite different from Bioware's style, but if done correctly it could provide a really fascinating story with a tonne of replayability. Especially if choosing Character A would lead you to, say, one of two possible endings because of their personality, while Character B would branch out in a different conclusion due to a different personality. Maybe choosing Character C would mean Character A objects to their leadership and either leaves the party or fights you. There could be an unparalleled degree of character interaction and tension.
It's got loads of potential, but I don't really see it happening. I think Bioware would deem it too likely to alienate their core playerbase (and they'd probably be right). But maybe a different developer picks it up, if they haven't already, or Bioware could try it out if their current recipe for RPGs grows too stale?