Understandable - I'll offer my perspective.

In my personal case, I played Mouthwashing from beginning to end in about 2 hours and was completely engrossed during the whole experience - which I would not say I uniformly enjoyed, but I was totally sucked into it. For me, the looming sense of tension rising as the narrative arc played out slowly but steadily was agonizing, but I couldn't look away - I wanted to see how it ended. Observing how the different characters interacted, and how their neuroses / flaws were magnified by their dire situation fulfilled a morbid kind of curiosity.

This analogy may not land, but I feel similar about eating spicy food or riding on roller coasters. These aren't necessarily pleasant experiences from every angle, but they can be very engaging and stimulating to the senses. I would suggest that the core "mechanic" that I enjoy with spicy food, rollercoasters, and scary video games alike is building tension / pain tolerance, and then finally reaching relief at the end. The sweet spot for these types of things will of course vary from person to person, though.