You need Lex as much as you need Lois in a Superman movie.
Do you? Not every Batman movie features the Joker. We've had two successful Iron Man movies without the Mandarin. Lex Luthor is not Superman's only foe. It's just that the live-action Superman movies to date haven't been very flexible in their choice of villains. Just because something has been done repetitively in the past, that doesn't mean it needs to be done.