*They should be. At school, they can kick a ball, walk around or at least chat to each other. Outside of school they can walk in the woods, climb, explore, play in the river or the snow, ride bikes or skateboards…
The more we talk about this issue, the more parents will be encouraged that not “everyone else is doing it” and there is another way.*
DD and her friends do all that. When the bus isn't an abject failure, they actually get to school really early so they hang out in the library browsing books/magazines/chatting or in the cafeteria chatting,sometimes having a pastry. Playtime/Lunchtime they walk around ,chat,joke,giggle. Some days they do sporty after school clubs.
Not a lot of time to go out after school, especially with homework and not living particularly close, but they meet up a lot in half terms/weekends.
DD has organised a group meetup at ours (mix between primary and secondary friends), they went to the park, ran around, got some food ,back to the park. I'm part of it as well as the kids aren't all local so they needed parents to drop off/pick up. Other parents have different ideas about what's safe/what they're comfortable with their child doing.
Yesterday she did an impromptu bike ride with a friend in the afternoon.
She just organised to go swimming with a friend tomorrow.
Last summer she was out nearly every day on her bike/scooter or on foot or with a football.
In the woods, in the field, park, astro, skate park, tennis courts.
She arranges all this on the phone. She can keep in close contact with new friends and (particularly) old ones on the phone. It's a lot easier to do these things and you can be in contact and "be present" and build/strengthen/maintain relationships with a phone. It just is.
It's not like when I was a kid and everyone had a landline , and everyone went to the same local school anyway, and we were all out outside to begin with so there were always kids to play with. All that and Still managed to wrack up some monumental landline bills for my parents trying to keep in touch with my school friends.
And if you are in that kind of situation, an area with plenty of green space, where all the kids go outside, they all go to the local (close) secondary and know each other , then by all means crack on without a phone. It definitely makes saying no easier. But don't assume that's the case for everyone.