OP I feel your pain - and that of others who are in similar positions.
However while I feel BigDaddyFox may have not been very kind with parts of their comment, I think this part is key
Parents have other commitments, childcare needs, health issues etc. Should their child not attend? You seem to be working under the negative assumption that parents who dont volunteer are lazy
Please can you all stop judging parents who can't help out. And please don't try to enforce parent rotas.
Yes - I know that some of you have busy stressful lives and still manage to help, but some people genuinely can't. The fact that you can, doesn't mean they can.
I work full time (long hours) and am a single parent. I have three kids who between them do 10 activities a week. I can't possibly help out at all their activities even once or twice a term. And I don't think it's fair to have children deprived of activities because their parents cannot or will not help.
The idea of "you can send any other adult" only works if someone has another (willing) adult to send. Many people don't.
"You can bring your other kids" only works if your other kids don't have activities. Or homework. Or bedtime. Or behavioural issues. And for some kids their time at brownies/cubs/swimming/gymnastics is their space away from their siblings.
Anyone who works long hours or who has multiple commitments may simply not have the time to devote to volunteering - however much they may like to.
Parents with disabilities (including hidden disabilities), mental health issues, chronic illness may not be able to volunteer no matter how much they may like to. Should they have to give full details of their condition to their children's football coach so that the football coach can make a decision about whether this excuses them from "volunteering" or otherwise their children will be kicked out.
Lots of services in all aspects of life could not run without volunteers- the food bank, many hospital wards, the Samaritans, Rape crisis, Women's Aid, parkrun, AA, community mental health groups. There's a local countryside station near me which could not run without the local volunteer group. Should it be expected that everyone who uses these services puts some volunteer hours into them or they are no loner allowed to use them? Even if that isn't where there skill set lies? Even if the thought fills them with dread and fear?
You an many others volunteer because you enjoy it and because you can find the time to do so. That's not the case for everyone.
I completely agree that parents need to stop whinging and demanding. If they don't like what's on offer then they should definitely take their child elsewhere. But please everyone, please stop condemning the parents who don't volunteer, or try to insist that they should.