They are balls and because of their shape, they create less friction than something that is flat or has edges. Edges tend to dig in or scrape against other edges. The roundness of ball bearings allows them to roll with the objects.
Rolling friction is much lower than sliding friction. Ball or Roller Bearings always have much lower drag than sleeve bearings except for pressure fluid suspended sleeve bearings like in a car engine, but; it takes much more power to pressurize the fluid so it is essentially drag.
