Our minds have adapted through evolution to be afraid of venomous things in the same way that they've adapted to be afraid of large animals with pointy teeth or being on a tall cliff. The fact that not all spiders and snakes are venomuos doesn't change that basic disposition any more than the fact that not every lion you meet will try to eat you or that you won't always fall off a tall cliff. It's just safer (i.e., improves your chances of reproductive success) to be afraid of all of those things.

Some other insects/creepy crawlies are potentially deadly (e.g., bees) and others carry disease or are parasitic, so we have evolved a general dislike of insects/creepy crawlies as well. Fear Factor always has a stunt based on this natural fear.

We're not slaves to our genetics, however, and if we realise that a particular species of spider is not poisonous we can overcome this basic fear for that species. The innate fear is just so strong that most people don't manage to do this.

A phobia is a fear that is irrational. Fearing spiders and snakes is not in itself irrational, but if you are 100% sure the particular species you are confronting at the moment is not venomous then being afraid of it is irrational.

Treatments of phobias come in many shapes and sizes, but most involve desensitizing the person to the object of their fear, so that they eventually learn through repeated exposure that the object will not harm them.