That's like saying that electrical circuits are the reason that a web browser shows a web page when click on a link.
There's no "home button transistor", but there may be transistors that, by their switching, control the appearance and action of the home button.
Mechanism isn't rationale.
Web browsers work because they were designed at a high level to accomplish a task, and a lower level of primitives was assembled (literally and figuratively) to implement the task.
When we talk about reasons for something, we normally have intention in mind, and we're talking in a high level abstraction mode. Mechanism is just an implementation detail. In the absence of a designer, mechanism doesn't cease to be an implementation detail; we don't have legs because of the signalling actions of cells in the embryo, triggered by DNA and proteins, cause fleshy buds to grow into bony appendages. We have legs so we can move around, and we can move around because in our ecological niche we're more successful moving around. The reason for legs doesn't lie in biology at the organ or chemical or physical level, because those are implementation details to the reason mode of thought.