Reachability
Reachability is a fundamentally physical problem. When you're using a mobile device, your thumb can only reach so far before you need to rely on your other hand to stabilize your device. Mobile interfaces keep this in mind, designing interface elements that are built to be interactive on the bottom-half of the screen.
While it seems to be a small detail, many users operate their phones with a single hand. Using the top half of the screen for readable content and the bottom half for interactive elements is a design pattern that is built to be accessible for both people on the go and people that are limited in their dexterity.
It allows interfaces to be built for the widest range of users possible, fitting it neatly into the principles of Universal Design.