This question couldn't be more pertinent, given the distinct rise in PG-13 entry-level horror movies liked M3GAN, Five Night's At Freddy's, and Imaginary.
Most people call something a horror based on their personal experience. A good example of this for me is that while the game Outer Wilds is an action-adventure, I personally felt it was a horror because it tapped into a primal fear of mine that I never knew I had. By all definitions, though, it's not a horror at all.
To be truly designated a horror, things must follow certain conventions. It must have a threat that can't be reasoned with, true danger to any character as they seek safety, and usually induce false senses of security. The best horrors tend to be set in the real world and modern day because they're the most relatable, but this isn't set in stone. Just because something doesn't scare you or it's poorly made doesn't make it less of a horror, it just makes it less horrific.
Often, people decry something as a horror simply because they want to brag or gatekeep, as if not finding something scary is some sort of achievement. If anything, I envy those more scared at the basic horrors. That means the media is working and eliciting a response.
If you haven't watched this video, there's a great discussion around desensitisation and the merit of these beginner horror movies:
https://youtu.be/97cnZGc0UUE