Why do the collector brushes wear unevenly?

I will describe three options for uneven brush wear.
1 As you know, the rotor core is pressed onto the shaft, and the shaft is mounted on bearings. Due to bearing wear, or a defect in the assembly of the electric motor, a backlash occurs between the bearing and the shaft, and, therefore, the axis of rotation of the core, relative to the stator axis, is also displaced, that is, in some place the rotor will be closer to the brushes, the contact plates touch the brush and erase it.
2 Brush problem. Or rather, with a pressure device, the very same spring that presses the brushes against the collector. Somewhere it presses more, somewhere less.
3 After unsuccessful repairs. It happens that contact plates that have undergone a rough cleaning have protruding sharp burrs or just an uneven surface, so much that they grind down the brushes like a blade.