I think that's a huge mistake: there are child-development studies that suggest that it's important for kids to engage in (moderately) risk-taking play in order to develop accurate assessments of their own skills and calibrate their own internal risk models; other studies suggest that it's helpful for children's social development to spend time playing independently (of adults) in age-diverse groups of other kids. My opinion is that the parents who (e.g.) follow their kids around the playground, making sure they don't fall down and settling every squabble, are not being helpful - though not doing that sure got me dirty looks from other parents!

So, that's where at least some of the "kids aren't exploring outside" effect comes from. My own (GenX / "latchkey" / elder-millennial) cohort was arguably under-patented; now we ourselves have over-corrected with our kids. Maaaybe the idea that at-home entertainment is today more compelling or convenient has something to do with it, but I don't know: my generation was plenty compelled by TV and 8-bit Nintendo. I think we'd have spent just as much time on those as kids today spend on whatever they do on their devices now.

However, I think the primary drivers are:

1.) Fewer kids around, both per-capita and (related, but separable) per geographical area: you can't have a spontaneous neighborhood "gang" when other kids don't live nearby. Declining birth rates and increased sprawl force parents to be more involved in their kids' social lives, as transportation to / organizers of scheduled activities and formalized "play-dates". (Of course, this only increases the time-pressure on parents, and makes it even more difficult to imagine having more kids. It's a vicious cycle.)

2.) As you say, experience-maxxing. Some of this is great: parents in general seem to be way more (emotionally healthily!) involved in their kids lives than my parents' generation ever were.

(But one example: I think my parents might have attended one or two of my Little League or high school baseball games, ever. It didn't hurt my feelings at all: parents mostly didn't, and we kids mostly thought the families who came every time were kinda weird, and were a bit embarrassed for the teammates whose parents showed up. Today, I love how much support my kid and his peers receive at their games. It's fantastic. [Coming to watch practice though? Nah. That's a little too helicoptery. Parents, don't do that, unless you're volunteering to help coach the team.])

On the other hand, I think a lot of the other efforts parents make - structuring every afternoon and family weekend around doing something kid-friendly - isn't such a good thing: kids need a little boredom and a bit of independence to spark their creativity. But... For time-pressed parents, who genuinely and rightly crave interaction with their kids, those hours are precious: that's all the time you get, so the temptation to over-schedule is strong.

I think both of those factors reduce down (in large part) to time pressure. Having to work or train longer, and on top of that having to have more than one income, to be financially stable increases the (fractional) time investment, and then all of it compounds.

It's not a simple answer, but I think higher wages, better social and societal support for families, and denser built environments would all help to pull societies away from the current equilibrium. Developed countries are all well below replacement birth rates, while at the same time surveys indicate that most women have fewer children than they'd "ideally" want. There's something off kilter in the way we're living our lives, and I think it's downstream of the economic system we've collectively built.

(Sorry to hit you with these walls of text. I've spent a lot of time thinking about this since becoming a parent.)