I dont find it hard to remember lyrics from music and what not specially when your the type artist that drops ill/memorable punchlines and what not. Take Eminem for example. Now "to me" his fist few albums were pretty hot. and just played them over and over again and listen to them and over time that shit just sticks. I guess artist remember there stuff because its well...just that "there material" there work of art. I see for getting your lyrics as being just like forgetting your own seeds name.
Now the better question is how do Red and Meth remember all of there lyrics cuz them nigga stay fucked up...
"The earth daughter rest her head on my chest
Through the struggle we cuddle under half-moon crest"