Why do the Power Relays have 12 V and > DC coil rating ? Cannot they have lower rating like 3VDC or 6VDC?

Normally the Power Relays with 10 to 15 amp c/o have 12VDC or 24VDC coil operating voltage. The question is can they be designed to operate on lower , even 3 VDC , coil rating.
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