Ubiquitous cameras and social media surely play a role. Each and all of your drunk stupidities can and will get recorded and posted to your social group for eternal humiliation.
This I think is the key here. The traits I would associate with Millennials and Gen Z are all related to reacting to being watched all the time. And while alcohol can help a bit with anxiety, zero tolerance for just being a kid breeds a culture where doing anything that can be clipped is a bad idea. Basically, they are boring kids because anything at all that can be used against them is. I really feel sorry for them. If time machines existed and they could see how kids behaved in the past, I think they would destroy all their phones and computers immediately.