Mainly because they work at lower pressure and depend on gravity to maneuver fluids (and entrained solids), drain-waste-vent programs use fittings whose interior surfaces are as smooth as is possible. The fittings could be "belled" (expanded a little bit in diameter) or in any other case formed to support the http://valvesandfittings11223.blogrelation.com/26828/the-2-minute-rule-for-valves-and-fittings