Cap'n Kobold
Hero
No matter how tight the maths are, they can't compensate for the variations in the people who play the game.So what you're saying, game design is basically on the DM because the team couldn't be bothered to figure out a workable solution? In which case, why are we even playing this game? Why don't I just pull anything out of the Monster Manual and say "well, there's your fight?" Why don't I just tell my players their characters get no treasure because "it's rare and there are no guidelines?"
That might've worked in 1977. Doesn't work today. Players want better than that. DMs want better than that.
Do you know how many of my 5e campaigns have ended in TPKs? Practically all of them. The others end because it gets boring because there's no challenge. Two extremes. Because there is no guidance.
The reason that 5e CR seems to low-ball the encounter difficulties is because it is there as a tool for beginner DMs running encounters for new players with unoptimised characters and tactics.
There is a world of difference between that party and a group of experienced players playing optimised characters with a fair amount of magic items (or even worse, got to pick them).
Social contract suggests that such a group should tone it down if the DM is new, but otherwise it is assumed that DMs will improve just as their players do and will be able to compensate.
The maths in 4e was pretty much as tight as it can get, and even then there was variation in how well a party could handle encounters based on character design and tactics.