Because a game where there is a chance of 'oh LOL looks like you just lost GG" is not usually a fun game. Imagine Dwarf Fortress, but every tick there is a one in a hundred thousand chance of the HFS tunneling into your fort. Would you enjoy playing, knowing that statistically all your forts would fall within a hundred thousand ticks, without any chance of stopping it barring extreme luck?
The funny thing is this used to be a mechanic, sorta! Maybe you're referencing it.
In old 2D DF, once you found any adamantine (or just mined far enough to the right, I don't remember) the fort had a chance to crumble, or maybe a timer. I think it was a chance per month, and based on the amount of adamantine mined. It didn't actually spawn the HFS, just destroyed the fort with a brief message.
(of course there was also the constant threat of save-corrupting bugs with the same result)
Dying in a typical FPS/stealth game usually just means a quick reload from the last save. The whole game is not ruined because of 1 death; it is not permanently lost.
Being able to reload the game is one of the advantages players usually have, but I find it very immersion breaking. I prefer it when games offer a penalty and explanation for death, even if it's kinda contrived.
In Doom, the player character is pretty well balanced with the enemies. They have crappy AI and mostly dodgeable or melee attacks, but they have the advantage in numbers. Also levels are designed to challenge the PC, often by releasing monsters in favorable positions to attack you. The monsters don't need to pick up health kits for the game to be fair.
Typical stealth games revolve around observing a small area full of very predictable guards, then "solving" it by making a plan to avoid an alarm being raised. In MGS you can actually see the enemy view-cones on a minimap. Which is an *incredible* advantage... particularly since their view-cones are like 45 degrees or less... But it makes the gameplay work. Losing the minimap is like solving one of those stupid mazes where the walls are invisible. Except in this case if you touch a wall, an alarm is raised, and the stealth gameplay is over until you reload the game or hide in a closet for like a minute.
Not all stealth games have minimaps of course. In the original Thief games you instead have to rely on hearing footsteps, and hiding in areas of darkness. Garret can see in the dark just fine, the guards only search for about a minute, and he even has utility arrows which make the game even easier... But it's still a difficult game, particularly if you go for the optional no-kill objectives.
I love stealth games (: I like Assassin's Creed also but it doesn't really fit in the stealth-puzzle genre.