It all starts at an early age, maybe 6 years old, when they are given their own cell phone so that they can call mommy whenever they have a problem. I can't believe how many phone calls my employees get from their children, during the work day when the kids are at school, and very few of those calls would come close to qualifying as an emergency. A previous poster nailed it -- co-dependency. 15 years later, when the kid graduates from college, they have no idea how to make decisions and live with the consequences, because they have relied on mommy and daddy to do all of the medium and heavy lifting.
At age 21, our oldest son was drifting without benefit of a "rudder". College drop-out, doing menial jobs, no goals. My wife and I told him to go down and visit with the recruiters, because we weren't going to subsidize his aimlessness. That was 12 years ago. Today, he is a career USAF NCO, and has earned commendations at every rank he has held (that's him in the avatar). Tour in Iraq during the heaviest fighting, and two tours in Afghanistan. Married to a great gal. He has accomplished all of this on his own, and he has made us proud.
Bottom line -- at some point, they gotta be pushed out of the nest, so they can be adults. As long as mom and dad are making decisions and financing their lives, the kids will never be full-fledged adults.
Caveat -- I know that there are young adults who lose their jobs and move back in with parents until they can get back on their feet with new employment. That's a whole different situation. Typically, those folks can't wait to get out of their parents' home again.