The differentiating factor to me is that backends can typically define the runtime they want to use, while frontends are beholden to run on what the user brings

The differentiating factor to me is that backends can typically define the runtime they want to use, while frontends are beholden to run on what the user brings. A backend from a decade or two ago can still run fine, because it defines what it runs on. It doesn't matter if CentOS 5 deprecated a feature the apps needs, we can run it on a CentOS 4 VM (not that anyone should). Frontend feels like it has more churn because it can't define it's own runtime. If Chrome deprecates a feature, frontends have to adapt to that. Because frontends have to adapt to their runtime changing, it becomes important to use a framework that's currently supported. You can't keep running on Java applets, because browsers won't run them anymore. You can't keep running on RandomJSFramework if it stops getting updates, because features may get deprecated in the browser. I think it has less to do with the average frontend vs backend dev, and more to do with the environment they operate in.