If a city uses project finance for a new bridge, why is the lender's risk tied only to the toll money the bridge earns rather than the city's overall tax budget?

When a city uses project finance, it creates a separate legal box for the new bridge. This box is called a special purpose vehicle, which is just a business made for one single goal. The lender only gives money to this specific business, not to the city itself. Because the bridge stands alone in its own box, t....