I'm sure the safety aspect (real or exaggerated) plays a part. When I was a kid, very few parents would wait with kids at the school bus stop or pick them up with the car. We actually felt sorry for those few kids 'cause their parents seemed a bit crazy.
OTOH now one sees few older kids (11-16 say) riding even on safe bike paths. I don't think many parents would forbid that but few encourage it either. In the old days, some moms actually made their kids play outside during the day so the moms could get a little peace & quiet.
BTW, skiing has also declined in popularity among young folks. 6 years ago I went to the local hill with some friends & their kids...the kids really enjoyed it despite the cold windy weather. But now, pointless to even ask the kids if they want to go skiing. Thanks to Phones, they've quit all sports/outdoor activities & their bikes have long since rusted away.