Why doesn't a newly activated service worker automatically take over pages that are already open?

Short interview answer

By default, a page stays controlled by whichever worker was controlling it when it loaded, so a page's already-loaded resources aren't suddenly served by different code mid-session; a new worker only starts controlling existing pages after they're fully closed and reopened, unless it explicitly calls clients.claim(), often paired with skipWaiting() in install, to take over immediately.