Question

why do middle water layers in a pipe flow fastest?

Answer 1

they are not touching the surface area of the pipe.

Answer 2

At the pipe surface, there is no flow, this is the "no-slip" condition for liquids. Viscosity (mu) produces a retarding shear that is proportional to the change of velocity (v) with distance from the wall (y), (mu)*(dv/dy). This shear becomes zero as the center of the pipe is reached, and the velocity becomes maximum.