Why are material design controls displayed on top?

I'm working on a new web application specifically designed for android devices and consequently it needs to be based on the Material principles though this question is about IA in general not only the material design.

I would prefer to display the navigation controls at the bottom of the screen instead of top to make them easier to reach. I'm wondering does google advise exactly the opposite (i.e. on top)? Especially on bigger devices I find this very unintuitive. Are there materials (i.e. a/b tests) that prove people prefer the navigation controls to be displayed on top of the screen?