DH is a scout leader. The excuse he hears most is 'I don't have time'. Which when he's worked a 70hr+ week in the past in a high pressure job and then done volunteering for something else and done two nights of scouting, he doesn't particularly thing holds up.
He isn't asking them to do it every week like him, he's asking for occasional help. The troop has had a rule since before he was there that if you dont help out when it's your turn and do the fundraisers, then your kid gets the boot. That tends to focus minds a bit. The group couldn't function without a strict policy like that.
It comes down to certain parents not wanting to put in any effort and then hiding behind the 'well I've got a very professional and difficult job' shite. As if DH doesn't.
Parents who make excuses, therefore, don't get a lot of sympathy most of the time (they aren't arseholes either, if there is a genuine reason why someone can't - eg health or family circumstances beyond control or certain emergency workers). But 'I don't have time' generally is not good enough.
The system works well. The parents sign up on the understanding of the commitment involved. There's a massive waiting list, so if that set of parents aren't up for it, the next on the list might be.
The biggest trick has been to make it a social group for the adults, with beers after sessions and to discuss the evening/plan future sessions. The group expanded over lockdown which is counter to the trend. They've now got more volunteers to help regularly, not just occasionally. Happy adults make all the difference as they feel they are getting something from it not just the kids.
DH is now doing less than he was which is something of a relief to me! And he's working less hours.