Completely unknowable and inhuman monsters have little for the PCs to touch base with - there is no hope of understanding them - they are an unsolvable mystery. I think a scary monster is scary because there is the potential to (at least partly) understand its motives, its reason for being, how it came to be. And something about that agenda, drive, or background has to be astoundingly terrible.
I'm going to have to at least partially disagree. The less the PC's have to touch base with, the better.
By your argument, none of the following should be particularly scary:
1) Jaws
2) The alien in Alien
3) The alien in Thing
4) The monster in Cloverfield
5) The terminator in Terminator
6) The spiders in Arachnophobia.
7) The blob.
8) The zombies in '28 days latter', etc.
None of these are knowable or understandable. There is no understanding them. There is nothing to sympathize with (or nothing left to sympathize with). They have an understandable terrible drive, but it is utterly inhuman and generally is no more complex than the understanding, "They want to eat me."
They get significantly less creepy the more they are explored and humanized. The Aliens in Aliens are less scarely than the singular one, because the Hive Queen is given a human motivation and something like a personality we can empathize with. It still wants to eat you, but its less monsterous. Once a terminator is humanized in T2, none of the terminators thereafter seem as scary. And humanizing zombies has even created a lesser version of zombie fetish in the goth culture now that vampires have been taken by a wider audience.
I think there is a role for psychological horror but usually then the best result is from getting the audience to empathize and then pulling the rug out from under them to reveal that the monster is truly hideous and that there attempts at empathy were utterly misplaced.
I think there is a role for tragic villains that you can truly empathize with, but I don't think they are particularly scary.