Tiny, I think because of reliability issues.
Small rolling element bearings are more fragile and harder to provide proper cleanliness and lubrication over the life of the clutch componants.
cost should be lower for a plain bearing.
A plain bearing will live with more abuse
A plain bearing will carry more load for it's size than a
rolling element bearing.
Since the pilot bearing rotates entirely (no movement between the bearing componants)with the flywheel/input shaft most of the time, a reduction in friction would only benefit when the clutch is disengaged.
A plain pilot bearing is nearly always reusable at clutch replacement time wheareas a rolling element bearing is nearly always unusable/destroyed.
The bushing type bearing is normally made of porous(sintered-not made from solid)material so as to be self lubricating.
Don't underestimate the seemingly low tech plain bearing.
