I had the same question. Marbel is valuable in today world because of the rarity , beauty and difficultly in producing it. Soulcasting it should dramatically reduce the demand and price of the marbel. However, lets say that mining marble is still a great investment, there still is a huge problem. They use the soul caster to add large deposits on the property. This is crazy! They will now need to use significant resources to mine and produce the marble. The should just soul cast the marble in a way they can sell it without as much production cost.
Even more, an advantage of soulcasting is that you can product beautiful works of art out of easily malleable materials like wax and than soulcast it into marble or wood. Simply said, even if you could have the raw material, you wold still need experts devoting a significant amount of time and resources producing the final product. In short the ghost bloods devoted alot of resources into a program that would end up providing a significantly more expensive and less useful product.