Hussar
Legend
Because we think about newer and inexperienced DMs.
There are a load of monsters is in the MM that are secret cakewalks and TPK machines which can ruin confidence in the game or themselves.
Given that 5e combat is very forgiving, I’m not really sure what “tpk machines” you are referring to. Given that a pretty constant and consistent refrain is that 5e is too easy, I’d guess that “TPK machines” might be overstating things just a bit.