Because that would mean Everything that ever hits that context-root will be handled by FacesServlet, a requirement that FacesServlet already knows it couldn't possibly fulfill (It obviously doesn't make sense).
To achieve the mapping you intend, use a .xhtml
mapping on FaceServlet
<servlet-mapping>
<servlet-name>Faces Servlet</servlet-name>
<url-pattern>*.xhtml</url-pattern>
</servlet-mapping>