because that would be pointless. Not only would it delay/ruin a title shot for one of them, but also gus is on a 6 fight winning streak, all of which have been either a finish or a dominating decision win.
I agree, Gustafsson vs Teixeira would be a stupid move for the reasons you stated. Gustafsson has already earned his shot at Jones and Tex is another win or two away from getting his. It's pointless to make them clash at this point.