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They are made from aluminium, therefore softer than a solid steel pulley.
Hydraulic / spring belt tensioners from modern days keep the belt tight like a guitar wire, it vibrates when you knock it with a finger. There is zero compensation for the slightest lack of balance in a pulley, the belt pulls so tightly that even a hundredth of an inch of wobble in a pulley is transmitted away. This is why clutched pulleys were introduced in the first place: allow the alternator to free-wheel at the higher rpm and absorb crankshaft vibrations before they gnaw through alternator bearings.
Back in the days of manual belt tensioners with bolts and brackets, there was some slack in the belt, it gave back half-inch when pushed by finger. This compensated the vibration and rapidly-changing engine rpm, one could see the belt oscillating up-down when running at idle.
Racing 1.8T engines may run with fixed light-alloy set of pulleys, no crankshaft damper, even with manual tensioner in the timing belt, but they run for short time, at high constant rpm and never in city driving.
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