There's skills that require you to control Ancient Fairy, Black Rose, Life Stream, Power Tool and RDA. Heck, even Jack have a skill that help him to summon Red Nova. Now explain why they won't give Stardust?
The skill "Assault Exchange" requires you to actually have the respective monster (I.E, Stardust Dragon and RDA) Summoned in order to actually use it, meaning that they get the signer dragons because that skill is usesless otherwise.
stardust will either be his ace or a high level reward or multiple levels because it is needed for some synchro monsters. stardust isn't too op as there isn't a lot of monster destruction.