There is no reason. . . Because you are wrong. . In a fully compressed beam, the upper reinforcement plays the role of fixing stirrups, and the upper reinforcement at this time is the erection reinforcement. When continuous beams are reinforced, anti-negative reinforcement is generally set at the middle support to resist negative bending moment. The three ribs you see should be two in the corner, which play the role of vertical ribs and resisting negative bending moment. The third root should protrude in the middle of one third of the net length of the beam. (the net length of both ends of the bearing is large) also plays a role in resisting negative bending moment. Is based on structural stress. There is no rule that a beam must have several tendons.