A lot of places teenagers used to hang out don't exist anymore or cost a lot more than they used to. There's just literally less to do, and social media is what has replaced a lot of those gatherings.
Parents send their kids off to whatever babysitter they can for as many hours as possible before they get forced to parent. Phones are just the new babysitter.
These things point to bigger issues in the world today too.
Both parents need to be in work to afford children. Primary age children in school 7am-7pm 5 days a week. Plus in school holiday programs at the school during holidays. Working Parents are just too busy at work. When they get home they are all exhausted. The internet and tv does not require much energy to enjoy. We were all latch-key kids, it's probably illegal for a 6 year old and 8 year old to walk home from school and look after themselves until bedtime now. We were out swimming, bike riding, or hiding in the house pretending not to be home if anyone knocked on the door.
I'd disagree that there is less for teenagers to do these days - but that may be specific to my area. Almost all free public activities are family orientated. There are HEAPS of teen programs, far more now than in the 90s. Parents are given kidsport vouchers to pay for children to be enrolled in sports, there is a wider range of sports available to participate in too. There are free craft activities at shopping centres and hardware stores. Churches and youth groups still exist. Creating music is far more accessible now. There are more skate parks and totally excellent adventure parks that are way better than the maccas playgrounds of my youth. The teens in my town still hang out at the park, smoking, drinking and getting up to mischief (like stealing golf buggies) but now they do it with a phone in their hand. Sometimes filming their pranks which gets them caught... and that leads to the topic of viral trends and kids who have died trying them.
It comes down to choosing the lifestyle. I dont think there are many that are actively choosing how they want to live. I notice it BIGTIME with my school friends. They were absolutely perplexed that I could CHOOSE not to have children. I still get snarky comments from them about doing as I please. They didn't see it as a choice, it was just what they were supposed to do at that stage of their life. A duty of adulthood. And social media was a part of that. You know how people say pregnancy is in the water? Well as soon as one had a kid plastered on facebook, it was catching. They suddenly all had the exact same maternity shoots and pregnancy dresses, the same nursery set ups. They all do what their peers are doing. There is no thought put into it. Monkey see Monkey do.
Rather than plan for their 4 year old to build a city of blocks, they just do it digitally. Less mess, less fuss. Maybe they are lazy, maybe they are exhausted, maybe they just don't notice that its any different to a tactile activity. And then they dont notice who their 11 year old is chatting to online. And then we get shows like Adolescence to cause an uproar and be forgotten within a month because everyone got distracted by their phones again.
I dont mean to demonise the internet, I love it, I'd be very bored without it. I think its really cool when its not used to abuse. But I didn't have it until I was in my 20's, being bored was part of being a teen.