It's because they're made for normal indoor lighting applications, not plant growth. Normally you would want some mild side light in addition to down light. For that they work great. They're not terrible for plant lights, just not the best choice. You could just wrap them in aluminum foil or mylar as a quick fix. Though the light would still have to pass both ways through the plastic to get directed downward. I also don't think 90 degrees is the most appropriate beam angle for a reflector for plant growth anyway. 60 degrees is a nice triangle shape, 90 is like the corner of a square. Why would you need a beam with that wide a spread? If it was 400w you might need such a wide beam, to avoid having to put it 10' feet above the plants or whatever. But with little COBs run at maybe 50w each, I don't think height would be a problem.