(I'll keep this brief: I've written much on this topic in the past, and just want to give a brief overview).
Horror is a widely mocked and disrespected genre, but even mediocre examples of the genre deal with the biggest, most important themes.
Fragility, Frailty, Impotence, Mortality. Death, Evil, Psychology, Morality. Lack of Control over Existence. The Nature of the Cosmos. Sex and Misogyny. Authority. Group Dynamics. Individual Strength. Fear.
The most mediocre schlock in the horror genre is still about these things.