3 of 6 added more explanation

Don't forget that full flaps is a very high drag and very poor L/D ratio configuration for the plane to be in. By raising flaps the increase in tire grip for breaking is offset partially or entirely by the reduction in aerodynamic drag. There will be a speed threshold somewhere that will mean fully retracted flaps provides more breaking than full flaps, but don't forget you have to transition from fully extended to retracted giving you a surge in your L/D ratio. Remember that takeoff flaps setting gives better L/D ratio than no flaps at that speed and AoA.

All of those things changing during landing does not lend itself to smooth, predictable, and easier controlled deceleration. Plus, as others have pointed out, retracting your flaps partway through a landing distracts you from your task at hand, and for limited or no benefit or even, worse breaking performance.

Edit: The better breaking performance with flaps up may be true for some planes but not for all "light" tricycle single engine aircraft. This will depend on many design factors that the operation manual authors choose to use the KISS principal and just say always continue the rollout with the flaps up until you're off the runway or parked.

During rollout the wings may transition from flying to fully stalled due to the reduced air speed while on full flaps. This will be a sever reduction in lift, that greatly enhances tire grip, while at the same time has all of the drag as before. If the flaps are retracted in this sort of plane at that stage the wings may go back to flying while at the same time reducing the amount of drag (giving the surge in L/D). This surge will be particularly bad during the transition from full to no flaps. Even though there will not be enough lift to make any wheels come up, it will reduce the tire grip. That reduction in grip could cause one of the rear wheels to skid, suddenly reducing the breaking performance of one side of the aircraft and cause the plane to spin around. Depending on the speed your going when your nose gear touches down and you decide to retract the flaps, this could be very bad. Even if that extreme doesn't happen, the the loss of tire grip at the same time as a reduction in drag from the flaps means less breaking performance.

In the planes the OP flies their wings may remain flying (not stalled) for the "high" speed part of the rollout that aerodynamic forces are significant, so retracting the flaps severely reduces the lift and a bit of drag, giving the tires much more grip and breaking power than any loss of aerodynamic breaking provided by the flaps.