Because, there were some comics that were patently special, and which you would only swap for something equally amazing. For instance if you had this:
Well these were special issues even then, iconic storylines, and you wouldn't swap them for just anything ( or at all! )
But there were also less legendary ones, that were valuable simply because they were great.
Doesn't matter if you like Nick Fury or not, It's Chaykin AND Starlin. Not swapping that for anything.
Or how about this:
It's John Byrne at the height of his Bronze Age fame. Good look getting Dean Willetts to give you that for anything less than your entire collection.
But to your parents or a non fan, a comic is a comic is a comic. As a physical object they're all the same size and shape and they all kind of look the same. Which is why it stung your heart when your Mum treated them as anything less than the gold-plated treasures they were.
But equally there were comics that everyone agreed were completely worthless. So much so that you were actually offended by them, you kept them always at the bottom of the pile, and groaned when they reared their ugly heads.
On swapping afternoons, these were comics that people would ' do you a favour and take off your hands ' only if you added something good alongside them. Although more often than not, everyone else had been lumbered with that particular issue as well, so you had to keep offering more & more to get a sniff of the ones you really wanted from their collection.
As for a instance, The Invaders. As much as I like that series now, this was a completely worthless comic to us because it was Frank Robbins, so if you wanted something good you had to offer at least two Invaders AND something of comparable value to get it. And we all knew what the values were, they were written in stone. The Human Fly is another example. Everyone was, as one, of the opinion that this series was worth nothing to anyone.
Equally any series that you liked that suddenly got a fill-in artist for an issue, an artist that you hated, automatically became a real tug-of-war, as you didn't wanna have a run with gaps in it, but you also didn't want it contaminating the rest of your collection with it's stinkiness.
Carmine Infantino's '70's work at Marvel is a discussion for another time, but it's fair to say that when he took over Nova or Star Wars, well, then those books went to the back of the wardrobe as well.
There were, as I say, some comics that were universally agreed on as being so bad, they were actually offensive and you didn't even want to look at them. And yet we all ended up with a copy of it.
Here's one: Marvel Chillers #1 featuring Modred The Mystic.
There's more portentous dialogue than even Roy Thomas would use, and the art, though competent, completely fails to inspire. And what's with that chain mail legging and loafers combo for a costume?
I don't have to pick it up by the corner for fear of catching it's badness or anything, but it is the definition of the kind of comic we all found totally worthless as kids. Give you The Brute #3 for it?