Norton's Hulk was essentially standalone with no on-screen connection to the rest of the Avengers cast.
War Machine is a supporting character and the replacement had many more appearances with more significant roles over time.
Flash, Superman, Batman, and Luthor have all been main characters in at least one film. Replacing Affleck and/or Cavill, even Eisenberg and Miller, would create a break in recognizability for the universe for casual audiences.