It's only odd for people with external locus-of-control magical beliefs that things "just happen" to them. Phobias are irrational fears most people choose not to overcome. This is in contrast to people who aren't ruled by fear, and confront and overcome their fears. Therefore, a lack of agency argument would be moot. And, phobias aren't behaviors, they're dysfunctional, intrusive, reflex feelings related to particular triggers.