Why do secretion systems differ so much in their strategies? Why don’t all of the systems directly inject proteins into mammalian cells? How could a secretion system like some of the type IV systems...

Why do secretion systems differ so much in their strategies? Why don’t all of the systems directly inject proteins into mammalian cells?

How could a secretion system like some of the type IV systems secrete DNA when these systems mostly secrete proteins? How could you show that this explanation of DNA transfer by a protein secretion system is correct?