See, I'm not a proponent of "one hit kills you" systems. I dislike level 1-2 D&D because of how random it is: one crit kills you, and you can one-hit-crit enemies too. One lucky roll decides the outcome of the fight, not your tactics.
But there is a middle ground between "everyone dies in one, two, or three hits" and "every fight takes three dozen turns because everyone has hundreds of HP", and that middle ground is where FUN lies.
Every action on the battlefield should matter. If everyone can soak up huge amounts of damage without consequence, your turn by turn actions don't really matter that much. It turns into a battle of attrition where the one who can out-heal the other wins: if the enemy does 10 damage on average every turn but your cleric can heal 12 HP per turn, you will win, it's only a matter of time. Getting hit doesn't matter because other than reducing your HP (which can be recovered), it doesn't actually do anything. In a bloated system of attrition like that, fights that go on for a long time feel tedious and boring because you keep making the same moves but they don't have much of an impact.
For combat to feel engaging, your actions need to have impact. D&D is usally liked (especially the older editions) because most of the things you can do have impact. Baldur's Gate 2 gives your fighters some really OP weapons to pick from so even your weapon choice makes a huge impact: do you use Carsomyr, or the Flail of Ages, or Lilarcor, etc. Spells usually have major noticeable effects. Even some of the lower-tier spells like grease stay relevant throughout the game because their effect has a great impact on the battlefield: enemies walking over a greased area have a high chance of falling down. That's cool. Jagged Alliance 2 is another game that is often praised for its combat, and it also has high impact decisions. Getting hit usually does a lot of damage, there is locational damage (head, chest, legs), distance and lighting determine how easy it is to hit and get hit, so crouching in a shadow is better than standing in a light, etc. You will often try to maneuver your characters into a superior position with better cover and a better angle on the enemy. Explosives can punch holes into walls, removing cover for enemies hiding behind them, or creating new access points to rooms. Silent Storm, a game greatly inspired by JA2, takes this even further with its full 3D engine and everything-is-destructible physics. Enemy standing on a watchtower? If you have a rocket launcher you can blow up the foundation of that tower and make the dude fall down. Heard an enemy on the other side of a wall? You can shoot through the wall with a high caliber weapon. The game offers a lot of options, and most of these options are high impact due to the destructible environments and relatively high lethality.
The player needs to have options, and so does the enemy. And those options need to matter. A spell that knocks enemies out for X turns, making them unable to act for that time, is more impactful than a spell that merely slows them down. A rifle shot that can cause a wound is more impactful than a rifle shot that merely reduces HP, so wound systems are a great way of adding impact to combat moves.
The more options a system offers, the better. Flanking gives you an attack bonus! High ground gives you a damage bonus! Standing in formation makes it easier to block with your shield! Crouching behind an object provides cover!
If your combat system is simplistic and doesn't offer any of this, then extremely high lethality just means "whoever rolls the dice better wins", while extremely low lethality just means "you're gonna watch these two dudes stand there and whack each other for 30 minutes".
