With an SPA, it could be built to do this. But most SPAs aren't developed this way.
Take a look at the "RealWorld" example SPA showcase (it's like a TodoMVC for SPAs), which has 43k stars on GitHub: https://github.com/gothinkster/realworld
And go ahead and click on the demo site: https://demo.realworld.io/
In virtually every implementation, every time you change pages, the app re-downloads everything it needs for that route. Click on an article and then go back to the feed? Every time you will see a blank feed for about 500ms and only then do the articles stream in.
You can say "well it doesn't have to be this way" or "this is just an example". But the reality is that it usually is this way. This real world example is a good representation of how most SPAs are built, and they lack the type of functionality you're describing. They instead end up creating an experience that poorly emulates server side rendering, since now many page changes end up with a blank page or a loading indicator before the data arrives in a subsequent request.
I don't know about you, but I'd rather wait 250ms once and then have the page arrive fully rendered over a transition that takes 2ms and then 200-400ms of multiple requests finishing and the page popping into place. The latter is really annoying, yet it's the experience most SPAs give you.
