To my mind, your question is analagous to Why don't all artists just use one or two colors when they paint, regardless of what it is they're painting?


Every key is like its own "color" and presents the listener with a different range of sonic and harmonic frequencies, and tonal coloring...and, I agree with Barbara Shaw's comments.

If all songs ever recorded or performed were written in the same key, don't you think it would become extremely boring for the listener?

Different keys keep things FRESH for the listener.

How 'bout if the Beatles had recorded every single song they ever released on every album, in the same key? Think about that for a second, and perhaps you'll end up answering your own question.