I'd be willing to bet they can, they just aren't using it for everyday items. Throughout the Mandalorian show, we see beskar steel plates with the Imperial insignia melted into them, implying that Empire at least can mark their initials in beskar. They might not be able to forge armor, but they can make a spear and they can make their mark. Forging beskar requires not just heat but a mixture of electricity as well. One also needs a beskar gravity hammer and beskar tongs to handle the molten material, and I'm not sure the Empire has any of those. The forging of beskar is one of the most important secrets Mandalorians guards. It's a tradition that is written nowhere. It's passed down verbally from one armorer to the next.