"We" is the People, the Sovereign, the Government, whatever you like, whatever makes laws that allow for private property and regulates it. We suppose they have some divine insight into the ideal allocation of capital because empirically, so far, this system has shown to be the best way to allocate capital in many situations, so we allow it.

Outside of it, capitalist ideology also has the idea of the divine invisible hand of the free market, which is certainly an exageration, but in the real world markets with the proper conditions and regulations are pretty good at allocating capital.