I can't speak for everyone, but I think it's a) cliched, b) inaccurate, and c) lazy. There is no way someone likes "everything" with the exception of two gigantically-broad categories. "Everything" would encompass such niche genres as, say, medieval metal and shoe-gazer psychedelia and dubstep and classical sonatas and post-modern instrumental rock and modern lo-fi blues and grunge... it's very hard to believe someone would have such wildly eclectic tastes. More accurately, someone who says they live "everything but country and rap" probably really enjoys "every top 40 radio station except top 40 country and top 40 rap." It's much more telling for someone to give examples of what they actually like, or at least specific genres they actually like.
But that's just my opinion... you can like whatever you want!