I’m not a huge fan of the tone, but I’m going to explain earnestly anyway.

I own a cat for the first time as an adult. I’ve taken him to 4 vets, mostly for issues about his teeth. In that time, no one ever mentioned preventative heartworm/parasite treatment.

It wasn’t until I took my new kitten into the vet and they asked if I had a regimen. When I told them I guess I was embarrassed because I don’t, they said it isn’t necessary and lots of indoor cat owners never do it.

So, in the US, if your cat is indoor only there’s a chance their provider just didn’t explicitly recommend it like in dogs that are outside and interacting with foreign material on a regular basis.

I’m doing it for now because I don’t know if he is an escape artist, but if he ends up being a homebody like my senior cat, I don’t see any reason to be concerned about regular treatment against something that can’t easily come into the house.

So that said, if I hadn’t had a dog when I was younger I might not even know treatment exists for such a thing. Pet ownership is criminally casual in most of the world, and these things aren’t wiggly worms that scream ‘THIS IS A PARASITE’.