Surely most creators of SPAs are using some routing library, and I’m not aware of any routing libraries that don’t handle this anchor tag click behavior by default. It’s really a trivial amount of “responsibility” to take on.

Perhaps, but it’s something I encounter fairly regularly, especially when an unreliable network connection or other JavaScript failure breaks something with poor error handling. Again, not a showstopper but it’s a decision point: if you aren’t committed to testing this kind of thing on every release, you don’t have the resources to build an SPA.

It's no different than stuff like not trusting browser input or avoiding a massive session when you're working on a Web app: it's simply basic best practice stuff that you don't even have to think about if you have an idea what you're doing.