Why do accents seem to disappear when singing?

I've noticed in the past that bands like Led Zeppelin, The Beatles, The Who, etc sing (in a very melodic manner), they don't seem to have such a strong, if any, English accent to their words. However, when an English person raps or sings somewhat monotone the accent is clearly present.

What's the psychological/musically mathematical/generally scientific reason that the accent disappears when using a melody?