Why do you think that the micas (flat minerals) in slate, phyllite, and schist are all parallel, or nearly so, to one another?

Slate, phyllite, and schist are essentially foliated metamorphic rock types. These types of rocks are formed when they are subjected to the application of stress or pressure from the opposite directions. The foliations in these rocks are identified by the planar fabric, which is defined by the flaky or flat minerals constituting them.