The ballads are stories. Like any story, they have to be well-constructed, and good singers have always taken care over this. Modern singers pull together bits from different version, and even different ballads, to put together a compelling story. I suspect that singers in the past were no different.

even the longer ballads last only a matter of minutes. Even Carthy's "Famous Flower of Serving Men" comes in at under ten minutes, and most are shorter. If a story is well-constructed, and put across well is it really that difficult to listen to? People are happy to sit through dramas on radio, TV and cinema lasting several hours. Of course they need you to concentrate, in a way that other songs (which perhaps are not much shorter) don't.