The reflectivity can't just be good, it needs to be nearly perfect across a wide spectrum. At high power levels, even 99% reflectance isn't good enough and that won't be practically achievable for most weapons in any case.

The wiki link says otherwise. Silver and gold are <40% reflective in visible, while aluminium is ~90%, neither of that is enough. Reflectivity goes down very quickly as metals heat up.

That's true, but "reflectivity" even for a good first-surface mirror only means something like 95% energy return. That means 250W of energy being absorbed by an infinitesimally thin layer at the surface of the mirror, instantly pulling it off the glass and vaporizing it. Without assist gas to remove it in a cutting application, the cloud of plasma actually helps absorb a lot of the energy.