Why artists still perform old songs?

Well, why wouldn't they play it? The Clics never go out of style. Besides, where do you think the modern era music came from? Artists were inspired by someone's work. I'm sure you can look any artist, past or present, and find an influence which led them to pursue music as a career or what have you. I also suppose the playing of clical pieces is supposed to perserve the memory of the composers who wrote them. Mozart, Bach, Chopin, Beethoven--some of the better known composers live on in their music, just as Freddy Mercury or Frank Senatra, or Selena do today.