I wasn't going to get involved in this thread, but what the hell...
First: If you don't have children, or haven't raised children, your opinion on how to raise children and what the problem with children is, isn't worth SPIT. Not worth the breath you used to carry the vibrations forth. So do the world a favour and shut your yap, at least when it comes to raising kids. Ditto if you haven't raised your children in the last 30 years. Its a different world.
Second: Its not quite as easy as picking out one cause, because there isn't one single cause unless you say this: SOCIETY. Period. End of story. Children look around and see nothing but unhappiness, drugs, financial concerns, violence. It screams at them from devices and from TV and the radio, from people in bus stops, and from billboards. Ad nauseum, ad infinitum. Both parents have to work now in order to provide a home in a decent neighborhood. Single parents have to work two jobs or get inventive on living arrangements. Do you blame these people for being tired after getting home and making supper and cleaning up? I sure as hell don't. But then you have some dummy flapping their gums about absent parents, the kid hears this and then resentment builds. Sometimes. Some of the nicest kids I know come from single parent homes, where the primary caregiver has to work two jobs.
Discipline is gone from schools. It was eradicated in the 90s and 2000s by liberal nonsense. The idea that sticking a kid into "timeout" was plenty sufficent punishment for the worst of crimes. That teachers are no longer allowed to give detention. That every child deserves a 1st place trophy, even if they finish dead last. Some truly excellent teachers that I know can't wait for retirement because of the current batch of rules and idiocy they have to follow on a daily basis. Its too bad, because some of my favourite memories of growing up were of EDUCATORS who were allowed to do their jobs.
Drugs are now mainstream. Society and potheads teach us that its ok for 14 year olds to smoke weed and get high as kites. Forget the studies that show (I won't say prove, because there are still some whose opinions differ) that cannabis and other drugs, including some prescription drugs, and alcohol, are shown to trigger latent mental health issues in those whose brains are still developing.
I could on and on, and I'm sure piss off just about everyone on the site. So I'll stop now. But if you want to fix whats wrong with kids these days, fix SOCIETY. Its not JUST video games. Its not JUST bad parenting. There is no tab A that fits into slot B answer.