There is of course, a disconnect between the sense of danger and actual danger. The trouble is getting it across to players. Fragile characters offer tension, but at the same time they can with a bit of misfortune, undermine and ruin entire plotlines, generally disrupting play for a session or two. One other big factor is scale control, the designers tend to significantly underestimate the damage output per second in play, and how small bonuses become big bonuses when attached to additional attacks or multipliers.
There is of course, also the arguments from the realists that surviving those injuries are not realistic. But people can and do survive them in real life, without necessarily being the heroes of legend you play. Arrow and sword wounds tend to be fatal in the sense that you're down, losing a lot of blood, and stuff is going places where there shouldn't be stuff, giving you a bunch of infections even if you survive the blood loss. Access to healing makes up for it to a great extent.
Of course, in legends, heroes do die. They don't however, tend to die to random hazards(though they do fall to puzzle encounter absolutes like petrification), they do pick up permanent disabilities from conflicts, curses, crippling(and defining wounds), etc. They tend to survive the actual conflict itself, dying later of their injuries(which again, healing magic and a party would counteract).