So many sites rely on backend related functionality such as XHRs, session management and auth etc to render the front end. Sure I can cache and re-render the frontend from some historical snapshot but the same javascript that came down in the original load will execute again - with uncertain and probably not great results.

Providing functionality like that seems like a good idea on its face but implementing it would be yet another set of test cases and problems for site/webapp developers and nobody wants that in 2024 when internet connections are ubiquitous in Western, developed nations. In the case of non-developed nations, the money to implement such a thing just isn't a viable spend.

Doable? Sure. Worth the expense relative to other clear and present customer facing problems? I couldn't justify it.