Been there, done that... "it's all on the page, just check it mate". And suddenly you're that annoying weirdo that refuses to do things like everybody else.
This seems so weird to me. I help organize some local groups centered around hobbies and games, and none of them use Facebook because we know not everyone will be there, and we don’t want to exclude people. Sorry, but that musician group seems pretty poorly run.
No one has "bizarre insistence" on use of social media, they're just there and it's the easiest option for all parties. People like you and me are the ones who are perceived as the ones bizarrely insistent on not being on social media. This doesn't make it wrong, and it's luckily slowly becoming more and more accepted, but it is important to keep in mind.
You are right that ideally groups like these should cater to all audiences but that's a lot of effort, and many organizers do it on a voluntary basis, not as a job. In my case, I know that 98% of people are included in the media that I use for my audience, and catering to the last 2% would double my workload. Not happening.
As far as workload goes, we have not found anything lighter weight and less maintenance than an E-mail list.
Luckily, I have other hobbies that are less prone to this, mostly because they don't involve much event promotion if any at all and don't need any audience to exist. Example: my local shooting range uses a mailing list for communication and you can always hang with fellows at the club-house.