I never played it myself so I cannot tell from personal experience, but my guess is slenderman himself is not scary, but the way he appears is. When you walk around at night, everything suddenly appearing seems scary, even more with those spider-like appendages he sometimes has and the lack of face(=lack of recognizable facial features).
I tried that old game once, Arena, it was evening and I was in the dark sewer, there was almost no noise. I almost jumped the first time some stupid goblin attacked me from behind. It all depends on the circumstances.
They aren't scared, they're jealous. Society puts so much emphasis on images in this day and age that if you're even just a little bit overweight, you're considered to be fat. Of course we have Slender Man here who makes even the skinny people look like Roseanne.
It's a really good game ill give the people credit but like a few other people said it's really not that scary unless u play it really late at night. But other wise it's not so scary.
I played the game late at night, sound cranked with some friends, all the lights off, and I felt like I was going to have a heart attack, my heart was beating really fast. I was baked though, that might have had something to do with it.
Humanity has what seems to be an inbuilt fear of things unknown. Darkness itself adds its own, beacause our visibility range is greatly diminished without sufficient light, thus lowering our ability to spot and recognise potential dangers. Slenderman appears out of nowhere, has no facial features - thus being a major "wtf is this and what does it want?" and approaches us. Not being in a position to wait and analyse the situation the first reaction is interpretation of that as a threat and action basing on that..and since we're rather unarmed in there combat is not an option - so RUN! Later we don't even need him to be in sight. We still hold memories of a threat in the area and are ready to run if anything appears at all, just in case it's that threat.
That makes me remember the reaction of some fat dude to that scary, seemingly easy labirynth game - dodge, warcry(try to scare the threat away), ATTACK!..lovely fisthole punched through the monitor