This was my position from my blog:
Another major factor in YA and MG literature is allowing the characters to fight their own battles. It's a very common theme in YA for parents to be absent, abusive, oblivious, or otherwise useless. There's actually a reason for this, other than the obvious ready-made angst factor. You see, for a story to truly be centered around an underage MC, they have to do everything themselves. Parents, guardians, and other adults can't do it for them.
The protagonist holds the power. This may mean they disregard the wishes of authority figures, or have to pull themselves along on their own momentum because Dad's dead and Mama's a drunk. This works because if there's one thing a teen can relate to, it's being told they're not adults. That they're not mature enough to do X, Y, or Z. That they have no power.
To clarify, this isn't about wanting to go out with friends to smoke and get drunk. It focuses more around decisions about their own lives that teens want to make, but are held back from. Young adult literature is about putting those decisions in their hands. The police aren't figuring out your father's murder quickly enough? Take it into your own hands. Dad is an abusive ass who has no interest in sending you to college? Do it yourself. The kindly headmaster wants you to stay in your room like a good boy while he and the other teachers hunt down the monster that hurt your friend? No way.
The point I'm trying to get at here is absentee parents aren't really about the dissolution of family and teaching teens to rebel against authority, as is often argued. It's about putting power in their hands. This can be also be done with a traditional family unit with caring parents, of course. It's just something to keep in mind - adults can't be the solve-all. The protagonist has to be the catalyst for the action, always.
And aspiring_x made a very good point about how there are often mentor characters in books with absentee parents, and why couldn't the mentor just BE a parent? Here's my response:
Others have argued because teens are at the point where they're discovering that people, including their parents, are inherently flawed, it leaves them jaded about adults and their family life. Also, (good) parents are inherently driven to protect their children, which means sheltering them rather than turning them loose on the world.
There are certainly parents out there who try to strike a balance between taking care of their child and letting them make mistakes to learn from, but it's a delicate balance. If a parent gives a child/teen TOO much freedom, it's argued that they're negligent. Too little, and the ability for adventure is limited.
Mentor characters are often not parents, as you said. Because they're not parents, they're not charged with the child's safety and well-being the same way a parent is. Their goal is to give the child the tools to succeed, and then letting them do it. Like Dumbledore - he gave the students the tools they needed, and then he stood aside. He wanted them to find their own power. As arguably the most powerful wizard in the world, he easily could have solved these problems himself, but he didn't.
Parents are automatically given toward wanting to solve problems themselves; it's parental nature. It's difficult to write a believable parent that lets their child run into danger. Not impossible! Just incredibly difficult. Which is why I think the tendency to take parents out of the picture is so popular.
Now, I am not a parent, so I'm speaking purely on an abstract/generalized basis, rather than from experience. This is my interpretation of how the trend toward absentee parents works based on the literature I've read and studied. I *do* think there's something to be said for people coming from broken families and then writing about it (I myself come from a broken family, so it's what I know), but I don't think that's the core reason.
What say you?