What about community-driven languages that are rapidly evolving? In that case, would you say that the community expects developers to always gobble up whatever is thrown over the wall? (See: web frontend development)
> "as if it's our duty to relearn how to do X for the Nth time, this time with bells and whistles?"
There are a few areas of computing that are fairly stable, if stability of knowledge is your priority, you should seek one of those out.
But in many other areas of computing, our discipline is young and rapidly evolving. Some of that evolution is being driven by a corporate need to sell the next version. But plenty is driven by the rapid evolution in languages and paradigms. And why is that? Because we all know that programming can still be (much) easier, more reliable, more predictable, cheaper, and more fun. This is why many devs, myself included, like investing in "ephemeral" knowledge.
Honestly, I understand that the decay of hard-earned skills is distressing for many people. Not all fields have this characteristic. But in our field, it just comes with the territory.