To come back to your horror movie analogy, this actually does explain why thrill parks can sometimes struggle.
Generally movie studios will try to keep their movies with a lower age rating because statistically movies available to more people perform better at the box office. This is not always the case, but generally this is the case.
Horror movies have to try very hard to appeal to the more niche audience because they cannot get a wider audience due to their ratings.
Teenagers going to thrill parks likely spend less than a larger family (which may also include some teenagers!) going to a family park. Everyone can go to a family park, but not everyone will go to a thrill park.