Browsers can only navigate via URLs. Traditionally these pages are rendered completely on the server but SPA's can just as easily load the appropriate page based on the URL, it's just done on the client and requires routing to be setup.
Whether it's actually setup like that is up to the developers to do, as many don't use routing for all inner pages. If they don't put in that effort then there are no URLs to navigate directly and thus there is nothing for a browser to do to open a new window/tab. Routing is a solved problem and is purely about implementation.
Is this a serious question?
It seems like you're letting a bit of bias against the JS ecosystem invade your thought process here. There are countless solutions because they range from generic, such as a simple routing helper, to deeply integrated, such as react-router.
> rather than relying on built-in browser behavior
You mean the built-in behavior of navigating away from the current page? The single page application?
No doubt single-page apps have their benefits, but there's an awful lot of new hotness kool-aid being passed around. The JS ecosystem is still suffering heavily from the inner platform effect.