cause they are unnecessary and an extra PITA step if you break down and put it together every time you fly. Ive flown lots for over 6 months with zero issues, the only issues ive heard of are due to improperly tightening them before initial flight. Ive flown many flights never using them with no issues at all, performing lots of quick direction changing maneuvers, there is no maneuver you can make that will cause them to spin backwards and loosen them to where they will come off if you tighten them properly while holding the motor. Ive never heard of anyone whos done this have one come lose and ive personally flown over a hundred hours with no issues.