Small imperfections on a reflective surface will absorb energy, heat up, and cause damage, causing more imperfections which absorb even more energy and heat eve

Small imperfections on a reflective surface will absorb energy, heat up, and cause damage, causing more imperfections which absorb even more energy and heat even more rapidly. Positive feedback loop. The emitter gets around it by keeping the beam diffuse and focusing it onto the target downrange; you can very reasonably field a laser with a half-meter aperture and a one-centimeter spot size. Dispersal requires consumables. You leave consumables behind when you move and you have to move because the laser can keep going indefinitely, so it's a losing proposition. Consumables are heavy and limited.