Because someone’s need to hear biblical truth isn’t significantly altered by his basic demographic. Christians need to hear about God’s character, work, redemption, and commands. While there are some commands more specific to, say, parents, than non-parents, 95% of the Bible is written to a pretty general audience. My most basic need is to hear and apply the Gospel, not to navigate the waters of professional life as a single. I don’t want to hear lessons every week on being a single woman, especially if those lessons displace lessons on the bigger picture.
When churches divide everyone up by demographic, they imply that the biggest needs- the most significant ones- stem from each person’s circumstances. Those groups risk becoming primarily demographic groups, and Christian only secondarily. If people come to a group because it’s an empty-nesters group, it’s tempting to let empty-nest talk usurp the Gospel, for example.
I realize that not all DBM groups tailor their discussions/studies to their demographic. I know of some churches that give every DB Sunday school class the exact same curriculum. (That, of course, begs the question, “If we’re all studying the same thing, why do I have to study it with people who share my situation in life?” the answer to which may be this.)