Ball bearing reduces friction to a large extent than sliding friction. Motors use a metal ring called bush which slides against the shaft when the shaft is rotating. If a force is exerted on the shaft in direction perpendicular to the direction of the shaft, the motor stops because of the so much friction present between the shaft and the bush. Here's a photo to be clear :
... Whereas, if a ball bearing is used instead, the force won't be that much effective in that case because of the balls present in the ball bearing as they have to roll and not slide. For those thinking what a bush is, here is a photo:
(Photo taken from Google, edited)
Why is the bush not replaced with a ball bearing?