That friction equation comes originally from empirical experiments, not from idealised models. The explanation in fact relies on deformable surfaces. If you i

That friction equation comes originally from empirical experiments, not from idealised models. The explanation in fact relies on deformable surfaces. If you imagine keeping the normal force constant and reducing the apparent contact area, the pressure on the surfaces will be increased which will cause the surfaces to deform more, resulting in more microscopic points of adhesion.