Why do they last? Because there are still some people who love them. They will last for as long as we, and our descendants, make them last.

Why are they so long? Because in the old days people did not have a problem with attention span. They were not inundated with distractions. It was not polite to interrupt other people. People were aware of that. When someone started a story, in verse or song, they were allowed to continue. And people wanted to hear the end.

Today, if a song is longer than three minutes nobody wants to hear it. Look at the uproar when Don MacLean released 'American Pie'! Nobody would believe a song could last that long and be a hit. How many long songs have been released since 1971 that have been as long? Maybe Lightfoot's 'Wreck Of The Edmund Fitzgerald' (How long was that song?).

Modern humans don't take time to listen to music anymore. Most people just use it to fill in the background with pleasing rhythms and harmonies and most don't know the words.

Many popular songs don't even have enough words to fill a thimble. A good folk song is wordy. The rhyme scheme is complex. And the story appeals. Modern people don't want to be bothered.