Answer:
In the absence of non-conservative forces that is friction, the mechanical energy remains conserved. The potential energy changes to kinetic energy as it goes down the hill. At the bottom of the hill
the entire potential energy transforms to kinetic energy. In the presence of dissipative forces like frictional forces, some of the energy is dissipated as heat. So, potential energy does not convert entirely to kinetic energy.
If coefficient of friction was included, the graph would not be straight line but a curved as at the bottom, the kinetic energy is not equal to the potential energy at the top of the hill.