They no longer are. That's why the slasher genre has faded into obscurity, and why the law of diminishing returns has hurt all Halloween/Friday the 13th/etc. sequels.
You missed the point entirely.
I was talking about simply featuring a person with a knife in your film. Those things haven't faded into anything; They are still heavily featured in horror films. The Babadook, for example. This was to counter the argument that knowing what your movie monster does makes it not scary. People have seen men with knives kill people in horror films since at least
Psycho. The very fact that slasher films were created in the first place counters the notion that once you know what a monster does, it stops being scary.
You're talking about a film genre, which is an incorrect comparison because the comparison was to the alien as a monster, and the alien isn't a genre. It's just a monster. What bores people today isn't a man with a knife - it's the tropes of the slasher genre.
I could've made my argument with Ghosts and Demons and the point would've still been the same: Just because you know what a monster does, doesn't make it not scary.
Firstly, men with knives aren't scary. Its the character holding the knife that's scary and once that character becomes too familiar they are no longer scary. Michael Myers was a scary man with a knife the first time round but he hasn't been scary since.
Michael Myers is actually a fun example because you knew what he does after the first 5 minutes of Halloween. He stopped being scary in sequels, when people started explaining him away and telling people who he is
. He was scary when he had a vague backstory, and stopped being scary once filmmakers started making a point out of telling his backstory. This is very similar to another movie monster we know.