When running a roleplaying game as the GM you have a responsibility to make the players feel excited. The surest and simplest way to do that is to convince them that they are on the verge of losing and usually that means nearly getting killed. The trick is that if they are constantly about to die and you use dice to determine outcomes they are eventually going to all get killed and the campaign will end and that kinda wrecks the tension of the moment.
Old school DnD achieved this sense of imminent doom by setting up the game such that any random enemy could roll well and kill a character in a single attack. A single bandit with a bow was a terrifying threat because that bandit could easily roll to go first and kill two party members before anyone in the group got a turn. Lots of other roleplaying games were the same and I have plenty of examples on my shelf - All world of darkness titles are definitely on the list, as are Cyberpunk, Shadowrun, most or all Palladium games, etc. In all those games pretty much any combat was terrifying because you could always just die.
Games have been moving steadily away from that model. DnD did so very dramatically with 4th edition and I took a few steps further when I build Heroes By Trade. I wanted combat to be more predictable and I wanted that feeling of worry to exist without constantly offing people when the dice didn't go their way. I prefer a system with a lot more granularity between 'totally fine' and 'dead'. Beating that single bandit with a bow didn't feel particularly gratifying or heroic because reverse lotteries where you almost always get nothing but occasionally get a catastrophic result aren't much fun.
DnD Next definitely wandered back to its roots in this regard. Now it is significantly better in that people don't regularly start the game with 1 hit point (Hopefully you don't get hit... ever?) but there is still plenty of 'okay, the bandit shoots you with a bow, critical hit, take 18, you die'. It is certainly less dangerous than the old days but I think it is still too dangerous to be the ideal game. As a simulator it is perfectly reasonable for people to die to a single arrow but for roleplaying purposes I don't much like that and as an engaging combat game that feels weak too. You just don't get to make very many interesting decisions when the enemies kill you before you get a turn.
Real life is super dangerous and lots of things can instantly kill you. In the fantasy worlds I want to play in there is a lot more of the heroes being hurt, worried, and panicked and a lot less of them just slumping over dead without warning Wash-style.