Exactly the point.

Bread crumbs, carrots, onions, even celery. All filler elements to make the meatloaf taste better and go further.

It's a WW2 meal designed to make rationed meat go further.

How is that recipe not just a giant baked burger?

We used corn flakes, 3 eggs (had chickens, have them now too), Worcestershire, onions, and maybe celery, mixing it by hand was good fun to me for some reason. As an adult when I started growing mushrooms, I added a bunch from a flush that produced way too many to use and was starting to go bad, and omg I love it that way now! (Not necessarily mushrooms on the edge of going bad, but is a good way to use a lot up and freeze for later meals!)

It's also there so the proteins from the meat don't all bind together and turn your meatloaf into a meatbrick.

When you have 6lbs of meat you don't need filler.

My mom (and consequently, so do I now) would use quick oats instead of bread crumbs. Add in an egg, onion, green pepper, maybe celery. Now it's sounding really good, haven't made this in a while, thinking I'll have to do that this weekend.

I'm not at all opposed to meatloaf but you're absolutely right. It needs some vegetables for flavor and moisture, you should generally use individual seasonings instead of premixed packets so you have some control but if nothing else it needs breadcrumbs for stability. You have to do so much to this to make it good that it just becomes a different recipe

You should be concerned when a recipe says you can add 50% to it and not adjust the seasoning at all