But they have. or could have. At least Google (and to a smaller extend Microsoft), if you are using Chrome/Bing have exactly that signal. If you stay on the site and scroll (taking time, reading, not skimming) all this could be a signal to evaluate if the search result met your needs.
I've heard google would guess with bounce rate. Or another way, if the user clicks on LinkedIn website A, after a few moments keeps trying other linksw/related search. It would mean it was not valuable.