You are simply fighting monsters that are more powerful. In contrast in a good pen and paper game you reach a point where you become hard to kill. Where you can tackle most foes. Likewise the truly epic monsters are not so epic because players can make mistakes and die then simply get a ressurection from the nearest cleric and lose less than an hour of playtime. Not a big deal when you have months of playtime on that character.
Players keep trying over and over until they get it right.
There's simply not much to lose. They don't have to develop strategies so much before a fight. They will learn what to do after enough experience with this same monster. With permanent death the majority of players will be at the lower levels because the high levels will eventually die and have to re-attain this point of power. Rather than a world of players separated only by the amount of play time on character you will have a world reminiscent of the pen and paper games with many low level characters and a small majority of powerful adventurers.