So winning is a given eventually. Losses may cause setbacks but since the characters always make it through, they can can just try try again until successful. That kind of makes not only dying, but the whole process of taking damage not worthy of concern. Why should a character fear the powers of enemies if he/she KNOWS that it is only a bluff?
There is no need to heal wounds, or even worry about getting enough rest, food, or water because nothing can cause any real harm.
Even should the party fail to stop the big bad from destroying the world, its no biggie because while the world dies, they are OK because they didn't give Mr. bigbad permission to kill them.
I won't try to speak for [MENTION=6855497]MiraMels[/MENTION], but this analysis fails the reality test for many campaigns in my experience. Sure, this might be true for some players and groups, but it certainly isn't a universal law. My last epic group and I tended to think of the game along the lines of a great novel like
The Lord of the Rings. The story was epic. The stakes were high. There were setbacks and problems and unexpected events. A few characters died along the way, but the fact that the main characters were going to survive didn't detract from the power of the story at all. Indeed, the bittersweet ending of LOTR was something we felt was true for the game too, with so many sacrifices made along the way. (Even on rereading LOTR, I still
feel like everyone is going to die at Helm's Deep, even though I
know exactly how it ends.) The game was thrilling for all of us because we didn't know how it would end and we didn't know all the twists. We just knew that time invested in backstory wouldn't be tossed aside because you slipped and fell into a pit or got hit in the eye by an orc arrow.
The logic of player investment can be reversed. We've all likely seen players who don't invest much in their characters or the long-term story. Sometimes they take ridiculous risks because there's nothing at stake. After all, they've got a nifty warlock concept in their back pocket. That's a bigger hassle, IMHO, than players not investing because they think they have invulnerable plot armor.
In reality, of course, there are many styles of play and many types of players. I've enjoyed playing in and running many different styles and love hearing about styles that go beyond my own experience and comfort zone. I don't doubt that the groups are having a rewarding time, and that's ultimately what this is all about.