FacesContext#getCurrentInstance() returns null in Filter#doFilter()

Inside a

public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)

I wanted to get the session using

FacesContext.getCurrentInstance().getExternalContext().getSession(false);

But I realised that FacesContext.getCurrentInstance() returns null. I know that I can get the session using ((HttpServletRequest) req).getSession(false) instead, but my concrete question is: why is the faces context null in the doFilter() method?

I'm using MyFaces 1.1.