Barely making it through is more fun than casually strolling through
The way D&D is designed, you’ll very often stroll through the first couple encounters of the day. You have way more resources than you need to handle them, since a medium encounter is only designed to take like 1/6th out of you. The last fight or two in the adventuring day might be extra tense, but you have to do some filler first. I don’t like that. I’d rather have all the conflicts be meaningful.
Now, in 5e you could just run all deadly encounters, but this quickly creates several problems. Short-rest and no-rest classes don’t get to shine, for one thing. Secondly, if you’re doing a lot of long rests you can’t really have time sensitive plots.
You could instead run a different rules system that has the desired feel from the start instead of putting the round peg in the square hole. But as I said elsewhere in the thread, D&D is so mega popular it sucks all the air out of the room. It’s hard to find players for other games. Hard to find community discussion for other games. Plus, if you take someone who’s only ever played D&D and plop them into another kind of game, that’s often a difficult transition.