Laser scanning: Why are things like deflection and mirrors used instead of just a mount and servo?
A few thoughts:

Weight: moving a lightweight mirror will be easier than moving the laser assembly.
Speed: related to weight, the system response can be much faster.
Reliability: no flexing of wires which may fracture or break.
Infinite rotation: Mirrors can be made in a prism fashion to do a raster scan by continuous rotation. (See the checkout laser scanners for example.) With constant rotation speed there is no non-linearity caused by acceleration and deceleration.