I suspect that a lot of issues come into play with this:
1. Social media means a lot can be done without people physically getting together - organisations that don't incorporate that suffer and become dinosaurs (not everyone wants to meet regularly on cold Melbourne nights to do what could be done on line). But this requires websites, expertise and extra time from someone.
2. Legal liability issues and a litigious society, especially for office holders (you can go to gaol or at least have huge personal costs for situations that would not have been considered even relevant in days gone by) - some is no doubt justified, but there was more emphasis on the personal responsibility of individuals, so that accidents were not automatically the fault of others - "I cut myself on your broken window while breaking into your clubroom". New Zealand seems to handle this better I suspect. One of our kids' schools closed their climbing wall in the gym for such reasons. I ceased volunteering for some outdoor things with kids after finding organisations were not covering me for liability insurance etc. One that I retain requires lots of paperwork, courses and clearances (working with children etc).
3. Insurance issues and costs, need for an OH&S manual and procedures (not being judgemental but it did not exist in the past). e.g. anything involving people and horses, rock faces, chainsaws, vehicles, surf.....or being more than 30 minutes from a doctor (a university requirement for a while for geology field trips)
4. Need for incorporation, government regulation and all that goes with it (costs, paperwork and people to handle it).
5. Perhaps an increasing attitude from "the gov'mint" having responsibility for doing everything for you - "why should I give up my time for a working bee"
6. An urban society where some kids need to be told that milk comes from cows.
7.Costs - petrol used to go for a song (just back from the USA and hardly noticed fuel costs on a two-week driving holiday). Real estate and membership costs as a result. Most clubs cost an arm and a leg, even allowing for the real cost of money with time, the bob I used to take to meetings as a kid has suffered (your time was the main limiting factor, not membership fees).
8. Sport no longer compulsory in many schools
9. Less recreational time - youngsters have to take studying seriously and they need money to do it (our kids worked up to three jobs each). But when I was young we had nearly two days on weekends - once you grudgingly did the household chores, mowed the lawn and helped dad change the oil, but there was still nearly a day left each week.
10. Shorter attention spans of the electronic generation, and a tendency to like being entertained rather than making your own activity (and could we have afforded the financial equivalent of an iPhone - a cricket bat in your street was shared - I shared with the kid who had the only TV in the street)?
11. Parents used not to both work in MOST families, and this has steadily decreased through time (dad or mum could run you somewhere, help at working bees etc)
12. Replacement and perception, perhaps the biggest issue - ANARE and similar things (that I belong to) pass their use by date, cultures change. And all of the above add up to change the nature of activities. That does not mean that youngsters don't do alternative things. My impression is that they do - just more local, more urban etc. Our own kids were a full-time job ferrying them around their activities and I see them doing the same with their kids, and I don't see less sport, gym, dance, music, acting etc.
So it may be partly perceptions and that we don't share the same interests (and I suspect that many clubs are only still running because they don't meet all requirements, don't know their liabilities, and nothing serious has happened yet).