When Melissa McCarthy, as the newly divorced, 40-something mom Deanna in “Life of the Party,” decides to re-enroll in college, my seatmate at a recent screening turned to me with a question about McCarthy’s choice of major: “What the heck is she going to do with a degree in archaeology?”But I’m troubled by a deeper, and more existential, mystery: Why can’t McCarthy seem to make a decent movie?Since her 2011 breakout performance in “Bridesmaids” as the loopy Megan, the actress has starred in...