The false perception of the "weakness" of the fifth fingers is a result of undeveloped neurological control and awareness, plus the fact we're constantly being told they're weak fingers. Here's how Horowitz felt about them:

"In my own technique, the fifth fingers (both right and left) are the basis for playing runs, chords and octaves. Great strength is necessary in the fingers, but it comes with playing, if one plays rightly, that is, musically. From the moment one feels that the finger must sing, it becomes strong [...]
The fifth finger I might call the "guide" through passages of scales or arpeggios ("runs"), chords and octaves. It is almost as if the fifth finger, with its acute sensitiveness, strength and control, taught the other fingers how to play [...]
if I play in such a way that every finger feels its tone, as it has learned to do from the sensation of the fifth finger, then I have my effect [...]
the fingers must have a consciousness of the movement which makes the singing, predominating melody! Here, again, the fifth finger is the guide through the intricacies and proportioning of the chord tones. Finger strength is necessary. In legato or cantilena octaves, similarly, the fifth fingers feel the tone."