Is it that things like mod, punk, hippy etc implicitly require a lot more commitment of your fundamental values to really identify with them? Like, being a punk is meant to be an expression of who you essentially are and to admit that you could one day move on to something else is to lessen yourself as a punk, whereas being into cyberpunk or cottagecore or vapourwave or whatever, it's basically acknowledged that it's a thing that you're currently into and could one day move on from.
Maybe, not sure steampunkers would recognize or at least like your characterisation of their scene as just more shallow than punk etc though that don't mean you're wrong of course.
The more I think about it though, the more I feel I was on to something. Punks are people who go to punk gigs, listen to punk etc it's a real thing they do cos they have a life and punk music is part of it. An adjunct to a real life, however it developments from that point, cos of how it starts It feels real to me.
Whereas steampunk people - I guess - read some books and thought "I wish the world had actually been actually like that at some point and I'd been around then to be a swashbuckling pirate captain of a souped up airship, leading a carefree but noble life elegantly robbing the rich - until my relationship with a mysterious pistol packing flame-haired beauty carrying a secret steam powered thinking machine drew me reluctantly into a scandal involving the royal houses of Europe and a sinister black clad techno cult who..."
In other words a basis in fantasy instead of a music scene. And after that maybe they start wearing clothes to fit with the books, maybe meeting other fans, but always following something that is not real - whereas punk is (or at least was) an expression of something that was happening. I think that's a big difference, could well be THE difference.