Why would you not want to be using a rendering library?

Because it's just not very useful in some contexts. I've seen web extensions which mostly query the current page, and it doesn't render much or even anything.

Not all pages are SPAs either. Many apps are the old request-response with some dynamic behavior sprinkled on top. jQuery covers that well.

This model is also quite compatible with the rising HTMX where the state/rendering is driven from backend and you just insert few dynamic pieces with JS.