Wouldn't that be the same for something that can kill you in a round? Like getting hit by a giant?
At first level, when an orc can kill you with a single attack, it does feel very similar to being hit with a save-or-die at high levels. The main difference is, since you're level 1, you don't have weeks or months of history with your character. It's like some no-name red shirt dying in the first scene, rather than the actual main character that you care about.
At higher levels, when an enemy needs to hit you three or four times before you die - even if it's all in the same round - it's different. First, you get a lot of tension building with each hit, as the DM rolls the dice and you get to process each attack as a discrete event. Second, the outcome of a series of events approaches the average as you increase the number of events: if some orc crits you (or you fail a save), then that was one bad die roll and there was nothing you could have done about it; if a hydra bites you four-out-of-five times, then it's because your AC was low, as a direct result of your choices. Dying in one round against multiattack is both more dramatically satisfying and more "fair" than dying in one round to a save-or-die effect.
I definitely see where you're coming from, but I consider the threat of an instant death part of the appeal I guess. Also some spells and monsters seem to be so weakened without their save or die attacks.
A lot of monsters in 5E are underwhelming, and this is part of it. Another part of it is that none of them can inflict any damage whatsoever that can't be fixed with a nap. A death knight is literally incapable of cutting anyone unless it's a killing blow on someone who has already been worn down over the course of a long day. The healing rules in this game are shenanigans.
There are ways to make the monsters scarier without resorting to save-or-die, but there are also ways to implement save-or-die such that it's interesting and fun to play. The classic example is with the medusa, where you always have a choice of whether you want to look at it (and maybe die) or suffer the consequences (you're effectively blind for the round, but at least you're not dead). It's just when they take both approaches, to give you the choice and then also make it slow-acting petrification, where it really loses the danger.
I'm glad there's still a few monsters who can kill instantaneously. But I approach D&D from more of a simulationist than a gamist perspective... it's one reason why I hated 4e.
That argument doesn't really apply here. There is no real medusa that you're simulating, so it's not a better simulation for its gaze to petrify you instantly or over the course of a minute.
If you want to say that it's silly for a medusa's gaze to petrify you over the course of a minute, or for a banshee's howl to knock you unconscious without ever killing anyone, then that's more a matter of taste than anything else.
