Why are loop (Chicago) vents typically built with three elbows?
From the International Association of Plumbing and Mechanical Officials (IAPMO) "Uniform Codes Spotlight" archive from February 2018:


  The configuration of the return bend using 45 degree and 90 degree elbows purposes to maintain a vertical angle (see Section 224.0, Vertical Pipe). This is to protect the return bend from accumulating waste and to prevent waste from flowing down the return side of the bend if a blockage occurs.