The first build out of fiber to the curb is expensive. 2-N is cheap if you do it right.
Or maybe you're saying that private ISPs will perform the deployment and then share their fiber? Which seems wildly unrealistic.
firms specialize, it's an important attribute of a functioning free market. If there is demand for running fiber through conduits, local firms might specialize in that type of construction, especially if local knowledge and working with local governments can be leveraged across customers. Many industries are not vertically integrated where specialization is more efficient than vertical integration. (i.e. Ford stopped making tires and glass)