long songs (like someone already mentioned) depends on the style of music....most early hard rock had long songs...Led Zeppelin, Black Sabbath, etc, they all had lengthy songs..of course that was during a period where rock music was being experiemented with...
but nowadays long songs usually are written by metal artists or progressive artists...i havent seen a song written by a rock band that was over 6 minutes long in years....many have already explained why people write songs so long...to go from one end of the spectrum to the other in one go...
the reason I write songs that happen to be long is because theres alot i come up with that belongs in one song...to put it in another song would make me too redundant...also its an experimental thing...how would this fit with that?