Why do tarantulas, such as the Brazilian Black, typically resort to biting humans?

Because they feel trapped, squeezed, or startled

Tarantulas are not predatory towards humans and do not actively hunt people for sustenance. A defensive bite is almost always a last resort, occurring when the spider perceives an immediate, unavoidable threat to its safety or liberty. The most common scenarios involve the animal feeling physically constrained—trapped in a container or squeezed by handling—or being suddenly startled by unexpected movement or contact. Handling the spider improperly, especially when it communicates stress signals, often forces it into a cornered position where biting becomes its final means of self-preservation.