They were all approaching, or past, the female star's danger age of 35 (depending on whether you believe Crawford about her birthdate). With Garbo, like Haines, there were other factors - specifically, the war cut off the European market that made her expensive vehicles financially viable, and her departure was meant to be temporary. Shearer turned down Mrs. Miniver, which might have been a game changer for her, because she did not want to play a mother -- once you got cast as desexualized Mom, it was very hard to get out of such parts.
True. Spencer Tracy, another example of this new type of male star, joined the studio a few years later. The movies were changing with the Depression - fewer of the Ruritanian romances you often saw Gilbert and Novarro in, for example.