I think one of the major reasons is how easy it is for these guys to become friends now.
Sure the AAU circuit, high school All-Star games, 5 star camps have been around for decades. So has the "fraternity" of being an elite basketball player. But over the last 10-15 years or so, it's been so much easier to connect.
Ok, let's think about the '79 McDonald's All American game. Notable names include Isiah Thomas, Dominique Wilkins, James Worthy amongst others. Maybe Dominique, Isiah, and Worthy all hit it off. What are they really going to do about it? There's no cell phones, there's no Facebook, twitter, etc. Cordless phones aren't even popular yet and long distance rates are very real. So they're not writing down their home address and number on a piece of paper so they can write letters to each other (Dear Isiah, how are you? I'm doing well. Today I went to the park and put 50 on these jive turkeys. How is your summer going? Sincerely 'Nique.), They're not going to sit in their mom's kitchen calling each other on the phone every week. And then get a new number/address when they go to college and pass that along, a new one Sophmore year, etc., and then again when they go to the NBA. I have to think people's contact books were a lot smaller back then and filled with outdated info. So you meet the other players, but much harder to form those bonds when you maybe only talk to them once or twice a year.
Same thing 2 decades later when guys like Paul Pierce and Kevin Garnett are high school Seniors in '95. They have cordless phones (and maybe even their own number or cell phone). so they can at least talk privately in their bedrooms. They can even email. They might know each other a little better, but still hard to form those bonds.
Now in the next decade, everyone's got email and cellphones, texts, Facebook, Twitter, etc. Plus you have online gaming. You can meet another star player at camp or a tournament when you're 15, and be in daily constant contact with them, even though you might only see them once a year.
There's that video of Ben Simmons and Karl Anthony Towns playing video games together online. I don't think that bond/relationship for 2 non teammates was really possible 20-30 years ago. It's just something that really started happening in the mid-00's. It's so easy for these guys to connect and grow friendships now. That's a big part about what's behind the rise of the super teams I think.