From physics blog:

Maybe a better wording is "Why is the reflection specular instead of diffuse?" Wording aside, if this is about visible light, then consider the wavelength of the light (hundreds of nanometers) compared to the sizes of the atoms (fraction of a nanometer). I'll bet a good mirror is pretty smooth on a scale of hundreds of nanometers. A photon is a discrete quantity of energy, not an arbitrarily small particle-like thing. We call photons "particles" because we can count them, not because they're tiny. A photon of visible wavelength is necessarily spread over a scale of hundreds of nanometers. – Dan Yand 21 mins ago

Reply - "Specular" seems to be a key to the answer, and what you say seems to make sense. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm) en.wikipedia.org/wiki/Light

webelements.com/silver/atom_sizes.html
1000 pm = 1 nanometre (nm, nanometer)
Ag Atomic radius (calculated) 165 pm

A mirror is a smooth surface with minimal roughness at the microscopic level, so the majority of the laser light is reflected at the same angle of incidence. Some laser light will be scattered due to imperfections in the mirror surface. A single laser beam reflecting off of multiple mirrors could have a measurable difference in beam diameter coming off the last mirror compared to that exiting the laser.

Here maybe this will help

[physics.bu.edu]

Faulty perception. Laser light is highly concentrated, yes, thus when bouncing off a mirrored surface it appears to not have been diffused, but it has. In spite of it being highly concentrated (coherent) the photons contained in the beam still behave as photons. That means that when being reflected off a surface, each photon meets the surface at an angle, but the surface viewed under extreme magnification will show the the surface is microscopically rough, thus some photons will strike a surface slightly off angle and be deflected in a direction exiting the main beam. It is diffused.

Because the beam is exceptionally narrow, the majority of the beam will remain intact and it is not possible to notice the scattering. Fortunately, for anyone asking this question, they finally succeeded in capturing the flight of a laser beam as it bounces off a pair of mirrors. Note how the beam has spread as it progresses.

I don't know.