What good are death saves if you get swarmed and attacked again before you even get to roll one? What good is healing if you pick a fight you have no hope of winning? A crafty group of enough kobolds can TPK even a moderately powerful party if they use clever tactics and make sure to hit each person until they're dead (instead of downing them and then targeting someone else).

And sometimes RNG just has it out for you. I accidentally straight up killed a level 3 PC with an owlbear, who scored a particularly brutal crit (33 damage) on its second hit against the fighter. No death saves, just massive damage, immediate death. Part of the problem is, 5e instills this mentality that every fight is winnable, and that is reinforced by making combat how you earn XP. You are incentivized to fight even if the fiction is suggesting you run like hell.

I'm just saying, if the GM wants to run a deadly game of 5e, the tools are there. I think the problem is that the default assumption is "big damn heroes" so people default to running it that way. But like... if you're level 5 and you happen upon an adult dragon and you think "well if we couldn't beat it, the DM wouldn't have us fight it," you're gonna have a bad day.

Revivify costs a almost a year's worth of living expenses for the common person. Resurrection, over 3 times that, and requires a high level caster to boot. In a low-magic, low-wealth game, either of these are a huge deal. 1000gp worth of diamonds is a lot of money, even for a wealthy citizen (that's nearly a year's worth of living expenses for the wealthy). The GM can simply choose to make these resources less readily available.

My point is, "it is almost impossible to die" depends highly on the play style of the group (and to some extent, on the dice as well).