Answer:That only applies to highly polished surfaces, eg mirrors.
If you take a high quality laser (ie with low divergence) and aim it at a wall, you can see the spot where the laser beam reaches the wall from anywhere with a direct line-of-sight to the spot where the laser beam reaches the wall. This due to micro imperfections on the surface of the wall. At a microscopic level, the wall surface is very rough and pointing in all directions.
As to why, a beam of light bounces of a highly polished surface, I can only surmise that it is essentially due to kinematics, ie the only force opposing the light beam is normal to the surface, hence there no forces along the reflective surface. Since there are no forces along the reflective surface, the speed component of light along the reflective surface remains unchanged. However, on the plane perpendicular to the reflective surface the, the light photons bounce off at the same speed at which the hit the reflective surface because the mass of the reflective surface is much much much larger than the mass of the photons, which means that the reflective surface won’t move at all. Since conservation of momentum requires that momentum after the collision be the same as the momentum before the collision then the only way for that to happen is if the velocity of the photon perpendicular to the reflective surface is of exactly the same magnitude but in the opposite direction. Vector resolution of the speed component of the reflected beam means that the angle of reflection must be the same as the angle of incidence.
Step-by-step explanation: