Plenty of other animals are venomous or otherwise dangerous to humans, lots of animals are fast and agile, countless animals are predators with fangs. It's not what they do that makes them frightening. It's all about how they look.
Spiders are completely different to us. There's no connection we can make with them. You can't personify a spider the same way you can a lion or a bear. Spiders are just alien and that's terrifying.
We get on best with things that have a recognisable face, even if they're savage predators that will kill and eat you in a heartbeat. Two eyes, a nose and a mouth below that, two ears to the side of the head. The vast majority of animals most people can think of have that face. Mammals, fish, reptiles, birds, amphibians - even a lot of insects. We can imagine expressions on them, project our own feelings on them, it makes it easier to like them.
Spiders? Nothing like that. Too many eyes. Mandibles instead of a mouth. They don't even have the right number of limbs. Spiders are just so completely alien to us and that is what makes them so terrifying.
