I think the issue lies in the fact that "scary" can be hard to define. There are many synonyms which can mean subtly different things: startling, tense, disturbing. Plus we're all very unique when it comes to what scares us, and our past experiences and phobias largely informs this. Something that one person may find to be no big deal may be horrifying to another.
Take Dead Space for example. I would call that game "intense" because of the quick reaction times and reflexes needed to cope with the enemies, and also "startling" because of the propensity for said enemies to suddenly appear. What sets Dead Space apart for me however is the sense of decay and decomposition that permeates the game. There's this sensation of rottenness, not as in that game being bad, but as in a certain perversity or corruption in everything around you. Because of this Dead Space is a game I'd personally describe as "disturbing." All-in-all the combination of tension, knee-jerk reactions, and decay make Dead Space the videogame equivalent of an amphetamine binge.
Also I'd like to mention that in my opinion "horror" is not the same as "scary." Horror is a genre, and as such it's informed by a certain set of qualities that may not even include scariness. It's the same reason many old Universal Studios monster movies are still classified as horror despite their not being particularly scary to modern audiences. There's a camp-factor and a lineage that some people respect in and of itself, and even though something may not be overtly frightening, it can be recognized as belonging to the genre as such. Horror as a genre explores people's less pleasant inner emotions and questions the structure of day-to-day existence. It disrupts the comfort of daily routine, and it often triggers introspection which can lead to a greater appreciation for life.