In many of the posts above, there's an assumption that the inept and badly prepared singers are new, inexperienced performers who would appreciate an older singer's advice, or would be glad of a workshop to hone their skills. But in my experience it's often the other way about - the occasional new, younger singer will often be very good, or at least appear to be making an effort, whereas the worst offenders are older members who've been at it for years and are not getting any better, learning new songs, or even trying to improve. The club could put on a singers' workshop but they wouldn't be interested in attending.
As Shimrod has said above, it's laziness that's at the root of it, and I don't really know what clubs should be doing to address this.