I think it's also a question of swinginess vs speed.
Combat could easily be much shorter and faster if everyone - PC and foe alike - simply had a lot fewer hit points. Problem is, that also makes combats more swingy and less predictable; and while I personally would find this a good thing, the designers seem to disagree.
Longer combats - as in, having them take more combat rounds to resolve - reduce the swinginess and increase predictability by giving randomness more opportunity to regress to the average; at cost of said combats taking much longer to play through.
It's why I prefer
volatility rather than
lethality.
Lethality measures how likely it is that your character is going to die in any given situation. Volatility measures how frequently your character
changes state, whether or not that state is (specifically) death.
OSR games are extremely lethal in most cases, where literally any combat has a pretty meaningful chance of death until you get several levels under your belt, which you have previously noted may take
literal years IRL. But, ironically, they are not actually very volatile, because the state-flip is usually almost binary and happens hard, fast, and (often) without any ability to bounce back. Despite being a radically different kind of game, 3e also ended up in this space too, because its ludicrously powerful spells (particularly save-or-die/save-or-suck spells) meant that either you won spectacularly and thus didn't see much change of state, or you
lost spectacularly and thus only saw one change of state.
In a volatile but not particularly lethal game, the
risk of death is always there, but the actual
eventuality of death is rare. Instead, you have sudden harm and then a heroic rally. You have a bold maneuver and then a sudden cost paid. The experience is tense not because it could end at any second, but because the state-of-play is rising and falling all the time, and at different rates for different characters.
It's the difference between a bomb-defusing scene in a TV show (where characters are much more expendable than films or books, typically), and a chase scene. The bomb-defusing is tense because it
could go badly at any moment, but there's really only two things that can happen: bomb kills everyone, or bomb does nothing. You can't really milk it for any tension beyond that simple binary. A chase scene, however, can have all sorts of changes, up and down, back and forth. You can have an early setback and then a rally. You can have early success and then a sudden swerve (literally or figuratively!) And even the conclusion admits a spectrum of possible results, rather than a hard binary (even a "per-character" hard binary, if you want to specify that finely).