So part of what I'm seeing here is also the methods of how some DMs treat their games and their worlds... especially with regards to how game mechanics reflect the characters and the world. And that appears to be impacting whether or not backstories are encouraged and/or used for plot fodder going forward.
Some DMs here seem to suggest that characters who are 1st level mechanically are also essentially "1st level" in terms of who they are and what they've done. They are "new" adventurers. There is a direct line between their power and how much impact or control they have (or have gained) in the world the DM has put forth. Which is certainly one of the ways to play (and I think personally is probably the default way that most people have played the game over the decades.) You're 1st level, you are a newb adventurer. And thus you barely have anything you've done as a character that could be considered a backstory. And that's why a long or overly involved backstory isn't necessary nor desired, because your character shouldn't have anything much worth in their history to begin with. Anything of importance will occur once the game gets underway.
But then... there also seems to be those of us who don't necessary align game mechanical power to character standing in the world. I personally am one of those DMs-- I find game mechanics to be absolutely horrendous at actually representing ANYTHING within a game world. To me... the "board game" of D&D is merely that... a board game I and my players will play in fits and spurts in and around the story we are creating. And that board game can give us hints at what is narratively happening in the world, but is in no way a "1-for1" representation of what actually is going on. Because to think of the mechanics in that way makes for such illogical leaps in most adventures that I don't even bother anymore.
Case in point... some elf character has lived for 200 years up until "this campaign I've started" has begun and for some reason is only 1st level. Nothing they have done over those 200 years has given this character any power whatsoever. But now... they go off to "adventure"... they go with a bunch of people to delve in a couple dungeons, kill a couple monsters, and now suddenly they are 2nd level. And soon 3rd. And then 4th. They gain power exponentially... all quite possibly in a matter of weeks or days of "in-world" time (depending on the scenario being run)... for no other reason that the game mechanics allow for this gain of power to happen. For someone like me who DMs using modules and adventure paths... narratively, that's all a bunch of hooey. To me, I find it all stupid and exceedingly unfulfilling. But for other DMs? It's part and parcel for their game and always has been. And neither of us is right or wrong.
So I have absolutely no issue personally with a character's backstory seemingly giving rise to a character who should be higher than 1st level when a campaign starts. Because for me... "1st level" is merely where all players start the board game at and has little to do with the narrative. If your Eberron character was a former officer during the last 10 years of The Last War... great. But you'll still start at 1st level because the board game is easier and faster to play when you are there. And we will all be happier progressing through the board game by that start. And that's why I have no problems and encourage players to come up with stuff that they'd like to see reflected in the game. Because I care much more for how the players interact with the story we are all telling, and don't care if it is reflected mechanically. Cause mechanics to me NEVER do it justice.
Right now I have a player with a 250 year old dwarf, that started as a level 1 cleric. We simply explained it that he was in effect a priest without any special powers granted by his deity. Now he's serving in a different fashion and gaining levels as a chosen of his god.
Doesn't ignore his background it's just that not every cleric is granted spells, or at least not spells granted to a 1st level cleric. Another party member has also been around for a while but before they were pulled into the adventuring life they were just a blacksmith. There are plenty of careers that don't involve adventurer levels.