It's a core tenant of storytelling going back to the first stories ever told, through the great tragedies, to the next stories you see tomorrow.
Quick spoiler: It's also the end of every biography ever told.
Death is part of the human condition and the final chapter in every story... (except for comic books, but we'll get to that in a sec...)
And now we get to the portion of this conversation that's about the suspension of belief.
What you're talking about here is that you've read enough comics in your lifetime to know that ANYBODY can come back.
When I was growing up reading Marvel, there was a steadfast rule that TWO characters could NEVER come back from the grave: Uncle Ben and Bucky.
There was so much fan-anger from fans who were around my age and older when the Winter Soldier story came out-- because Bucky's death was viewed as one of the great untouchables. You weren't suppose to bring Bucky back!
But they did.
And you know what?
That story was GREAT! And, hell, it lead to one of my favorite super hero movies of all time.
And in the end, that's what it comes down to:
Telling stories to entertain. Telling stories that have you invested. Telling stories that affect you.
If you're going to play the card of "Well, I've read enough comics to know that anyone can come back"... I have to shrug at that. I don't mean to be cruel, but that's part of the price of suspension of disbelief. Either you're willing to go along for the ride and see where the story goes, or you can let your knowledge of tropes and convention hamper you to the point where everything can be picked apart, analyzed, and picked at to the point where you're no longer enjoying the story ON a story-level.
AND THAT'S FAIR.
I'm not saying that's not fair.
I'm saying that if that's the point where you're at, the medium may have lost some of its luster for you.
Take this example:
You KNOW we're not going to kill the main character in a SOLO title of an ongoing series.
Sure, you can bump off a Ned Stark (and, boy, is that shocking)...
...but for the rarest-of-rare times, you know that we're not going to stick Superman, Spider-Man, or Batman into the grave. (And even if we do, you KNOW they're gonna be back).
So you read their adventures every month with the knowledge that you're going to get to read them the next month too.
But you still GASP when they get into danger-- and hope that they get out of their next "inescapable" trap.
Even though you know they're going to be there next month and the month after.
This is no different.
If you can accept a character's death in comics and keep reading (even though you KNOW you're in a comic book world where anyone can come back at any time), you'll probably have a more enjoyable time. If you can accept that suspension of belief and keep reading, comics are fun.
As you get older, sometimes your enjoyment moves to another level. Maybe you DON'T have the same suspension of belief. Maybe you've read so many comics and understand the underpinnings of comic book universes, that this stuff DOESN'T surprise you or affect you on those levels anymore. Sometimes people keep reading NOT to be wowed by the magic trick, but to try to outsmart it and figure out how the trick was done. That's also one way to read and enjoy comics.
For example: When we did SUPERIOR SPIDER-MAN, we knew readers would expect Peter Parker to come back... but they wouldn't know HOW he'd come back. Enter Ghost Pete. Ghost Pete was there to tickle and tease people who were trying to machine Peter's return-- who were looking for clues and hints to how he'd pull off his big return. And there were a number of fans who wrote in that
that was their favorite part of SUPERIOR.
IF that happens (and I'm not saying it will, but IF...) then the question is: Was that journey fun?
Did you enjoy seeing Pete come back in Superior and how it was handled? Did you enjoy the journey they took in the Superman titles? (I know that I loved the hell out of Kessel's SUPERBOY version of that. To ME that alone justified the whole DEATH OF SUPERMAN journey.)
The journey to get there. The journey after. The possible stories you can tell. In the end, it all comes down to the stories. And if they're not doing it for you, or you're not getting the FEELING you want from reading the books, it's totally cool as a reader to stop and find a different franchise or experience to put your time and energy into. I was really into DOWNTON ABBEY for a season and a half... then I wasn't. As a kid, I blasted through every single Michael Moorcock fantasy novel till there were none left to read. I made it all the way through THE WIRE, but dropped THE SHIELD part way in. It's all okay.
He wasn't.
But his death is where the story went.
Everything serves the story.
Those stories are still there. They're in your longbox whenever you want to read them. There was no plot or kabbalah to screw readers out of money or to tank previous issues, anymore than Gerry Conway did something underhanded to everyone who read a Gwen story before the Death of Gwen Stacy-- or the Avengers writers did something underhanded to all the readers of the previous WONDER MAN series after they killed Simon in OPERATION: GALACTIC STORM. If you're not in a solo book where you're the headliner, you have a potential target on your head. That's part of the buy in to the Marvel Universe. Back when I was writing THE THING, I had Bill Foster as a recurring character. Then my title got canceled-- and CIVIL WAR happened. I guarantee you, if THE THING was still an ongoing at that point, Bill Foster wouldn't have died during CW. But that's the way it played out. And it wasn't a "screw you" to everyone who'd been reading my THING or to anyone who had a run of BLACK GOLIATH comics in their longboxes. It was just where the story went.