Spiders are one of the many things that represent fear of the unknown, and that is what scares people.
Mosquitoes will always suck blood, that's what the females do. Spiders, however, can spin webs, they can bite, they can just sit, they can hide, they can jump, and sometimes they can even catch fish. Both have the potential to kill you, but with spiders, you don't know at a far glance whether it is a black widow or just a harmless house spider. Mosquitoes will always come for blood, they are common with the summer evenings, and most people have experienced the red itchiness that shows up. Spiders are killed upon sight before they get the chance to do anything.
People are much that way, sitting there in the web they've made that they naturally know how to make and repair. They stay safe and secure every single night within their protections.
As a child, there was a "monster under the bed" or a "monster in the closet" but never a monster sitting right next to the pillow, a creature from a different dimension anyways.
And so the easiest way to quell those fears was to take a flashlight to the dark. What isn't seen, must not exist. But children still lie awake staring at the closet waiting for something to jump out at them.
Age didn't eradicate the fear, it stayed as the same root just in a different form. Still, the unknown. Yet it becomes more tangible. Someone can reach out and touch the person they love, but yet remain afraid they'd leave. The feeling of being alone and having to search blindly for someone else to hold, without the promise they exist, or will quell the fear like another.
There is a reason the weather is predicted, and it's because no one truly can tell with 100% accuracy what tomorrow brings, or what the outcome of the wings of a butterfly will create. When that notion hits, that is one of the first true moments of adulthood. The decisions made will have consequence, most unknown to the maker. That is scary.
Books are popular because there is a tangible end. It must be resolved. Another book may continue the story but there will be an ending and a resolution dispersing the clouds of unknown outcomes.
New things, different paths, open doors, are all like spiders. Not knowing which have the ability to kill, people run from the unknown back to their familiar haven and form barriers around their minds and hearts. By sheltering themselves from any risk is the method of choice, there is less risk to end up hurt beyond repair. Cautiousness is key. Yet they end up protecting themselves from both the crash and the rewards.
Most would rather stay in the same place, then take a risk. This stems from a lot of different places in someone's life. Too often the fear of unknown, fear of losing, keep others from trying to grasp something better or something they want.
Of course, there are many open doors, but without the courage to step through them, that's all they'll ever be.