The problem is that this is hard. Just think about how game engines consist of a graphics engine and a physics engine. You would need various other engines that

The problem is that this is hard. Just think about how game engines consist of a graphics engine and a physics engine. You would need various other engines that can just be plugged in. A realistic economy requires a complicated supply chain for example. This means just so your NPC vendors can sell a sword, you would have natural resources that can be extracted, manufacturing processes that require inputs and machinery/tools and labor and all of this for something that the player clicks "buy" on and never looks back.