Because the JSF techology build on the J2EE technology and the JSF framework must be started by using URL such as Index.faces.
Users can call Index.html like the code you mentioned and then the FacesServlet will be initialized by mapping Index.faces url to FacesServlet in your web.xml file.
But if you dont want to use refresh the function of HTML code, you can change your FacesServletMapping.
By doing that you can for example create a .jsp page under the file structure (/appName/jsf/pages/index.jsp). However by doing that users must know the index page URL