Because offline updates prevent leaving opened libraries, plugins, and files around; if a file is opened by any process, the update will still succeed, but the new file won't be used. You'd have to close and restart both those browsers, first, but GNOME Software doesn't know if a package is related to a running application or not.

There's a good explanation here: fedoramagazine.org/offline-upd…

Restarting and Offline Updates - Fedora Magazine

Why Fedora Linux has to restart for updates: Offline Updates, which keep your system up-to-date and reliable at the same time.

^{Kevin Degeling (Fedora Project)}