Because most of the channelers don't seem to be as creative as the legions of readers are. If Elayne or Egwene was there when Rand was captured, I tend to think unlike Nynaeve's initial thought, one of them might have put two and two together, marched to the Guardian and disabled it.

Ter'angreal also seem to be very durable, though I'm of the opinion that opening a gateway on it, carving it up with balefire or directing a flow of Fire into it in order to melt it down to slag might have worked.