Perhaps this will not apply to a tiny dc motor, but the concept is valid for mostly bearings.
Most bearing supported pulleys in car fail because the belt tension always pulls the axial in one direction. To avoid this the maker should apply some tension 180deg from the drive belt tension (another tension belt, or spring setup), but they don't, so you end up replacing things sooner than they really need to. Cost is the variable they do not wish to increase, etc. Quality bearings can run a long time with perpendicular force on axial, some can't and fail sooner.
As for dc brushes, maybe it's the way the brush dust falls down in horizontal vs vertical? I have seen some nasty insides from brush wear down, where that crud goes makes a diff, perhaps even making items wear faster?